An introduction to hacking Mozilla

This guide introduces the Mozilla project and provides an overview for developers interested in contributing to or exploring the various parts of the codebase.

If you find errors in this document, or if you want to contribute updated or additional sections, please contact Kai Engert.

What is Mozilla?

Mozilla is an open source project and organization to develop a cross-platform Internet client software. Since it is open source, the source code is available to everyone – although you have to follow the licenses as defined in the respective source files (a mixture of MPL, NPL, GPL, LGPL). is the name of an organization that provides an infrastructure to help developers in the project. is also the address of the central web site for the Mozilla project.


Mozilla is one of the largest open source software projects. The Mozilla codebase has millions of lines of code. Therefore, getting started in this huge project isn’t easy. The intention of this document is to give an overview about what you should be aware of in order to hack Mozilla. It tries to build a bridge between the many different technologies used in the Mozilla project.

When I started to look into Mozilla, I wished a document like this existed. :)


This document is primarily written for developers who want to be able to work on any part of Mozilla. You should have a good understanding of Object Oriented Programming, and especially you should be experienced with C/C++, as it is the main programming language used in the project.

However, if you intend to work only on a subset of the code, say only the JavaScript and XUL UI code, this document should still be helpful.

Scope of this document

This document will try to answer the following questions:

  • How is the source code organized?
  • Which technologies are used?
  • Where do I start?
  • How do the components work together?
  • Which tools exist and how can they help me?
  • What rules do I need to follow in order to contribute?

What does Netscape have to do with this?

From my personal point of view, in the beginning of Mozilla there was Netscape. At some point Netscape, the company, decided to give away those portions of the source code that they owned and that were free of others’ copyright.

This wasn’t the complete product, therefore the Mozilla project has had to write a lot of new code. In addition, many portions were rewritten. Some other components were kept and extended. That’s one reason why, in this document and when discussing the Mozilla project, you will hear the term Netscape.

C++ and JavaScript

As it is widely used in Mozilla, it makes sense to explain how JavaScript and C++ relate to each other in the Mozilla source code. C++ is a compiled language, while JavaScript is an interpreted language. JavaScript is most commonly known as a technology used to implement web sites. However, the developers of Mozilla decided that the Mozilla source code itself should consist of a mixture of both languages.

When you start the application, the C/C++ components start first. But in an early stage, a technology called XPConnect gets initialized that enables the use of interpreted JavaScript at runtime. In fact, a Mozilla browser distribution consists of both compiled C++ and uncompiled JS files.

Note that JavaScript can not be compiled to be executed by the operating system directly: we still use C/C++ for the back-end of the program; JavaScript runs inside Mozilla.

Also note, when surfing web pages which use JavaScript, that JavaScript code is executed within a sand box, and does not have access to Mozilla’s internal objects. Only those objects that are exposed by DOM (Document Object Model) are accessible.

Whenever JS is mentioned in this document, it is meant as a technology to contribute to internal Mozilla functionality. JavaScript is mostly used in those areas of the source code that care for user interface events. Most of the following document will explain the C++ aspect of the application.

NSPR – Netscape portable runtime

A primary requirement for software developed in the Mozilla project is that it must be cross-platform, i.e. it must not be restricted to any particular operating system.

While C++ is intended to be a portable programming language, this portability aspect applies only to general program logic and data structures. If you want to write software for a particular operating system, you need to use functionality that is specific to that system. Often, you want the same functionality on all systems, but in order to do it, you have to write specific software for each platform.

The intention of NSPR is to provide a layer between the OS and the Mozilla source code, to allow for simpler coding in other areas of the Mozilla source code. When you are tempted to use a C library function, you should check first whether NSPR offers a cross-platform implementation.


Mozilla Firefox is a multithreaded application. You should understand what that means before you try to code Mozilla.

NSPR provides an operating system independent facility to program with multiple threads. For example, all network data transfer happens on a separate thread, to make sure the user interface stays responsive while data is being transferred.

One requirement for C++ code you write is that it is safe for multiple threads.

Object oriented programming & Modularity

Mozilla C++ source code is intended to follow the rules of OOP, that includes building modular components, where access to internal data (variables) is only allowed/possible using the public interfaces of your classes.

In most simple C++ projects, this only means that you carefully design your classes to use public/protected/private as appropriate, but all source code is still available everywhere. For instance, at any time, you can change any class component from private to public, so it will be available at any other place in your project. This does not apply to Mozilla. It was decided that Mozilla should be even more modular.

The Mozilla source code is organized as separate components. While within one component, you have all the freedom as described in the previous paragraph for simple projects. You don’t have the same level of flexibility between components.

When components talk to each other, they only do so using well-defined interfaces using the component object model (COM).


The concept of interfaces is something that is used in the CORBA technology, for example. Both CORBA and Mozilla use a similar language to describe interfaces, which is XPIDL (IDL means Interface Definition Language).

In a CORBA environment, life is more restrictive and difficult, because you have inter-process and inter-network communication – something which Mozilla is not actively using. In a distributed CORBA environment, it is difficult to change the components of an interface, because you are usually unable to replace all running systems at the same time. If you want to change something, you have to define a new version of an interface, but you might still be required to support the old one.

As Mozilla is not a distributed application as of writing, it is currently possible to change most interfaces as the development process requires it. But because the Mozilla browser runs embedded in some environments, those environments must be able to rely on a fixed interface; therefore, interfaces can be frozen. This state is usually indicated in the interface definition. As Mozilla stabilizes over time, the ratio of frozen to not frozen interfaces is likely to increase.

One step of building Mozilla is automatically translating the interface definition files into C/C++ header files. That’s the job of Mozilla’s own IDL compiler, xpidl.

Besides of the methods and data members, interfaces have additional attributes. They have a UUID, a number to uniquely identify an interface. Interfaces can have the scriptable attribute, which means they will be accessible from the JavaScript code. A scriptable interface is restricted to only use data types for parameters that are valid within the JavaScript runtime.

XPCOM / nsISupports / nsCOMPtr

XPCOM is Mozilla’s own implementation of COM – the component object model. The XP in front means it is cross-platform (do not confuse this with XP as it appears in product names for a certain operating system manufacturer). The fact that it is cross-platform makes XPCOM more versatile than any other form of COM.

You should definitely read the introductory documents on XPCOM on To get you started, one could say that XPCOM is the engine that makes the COM concept work. This includes playing the role of an object broker.

Typically, an interface describes an object that can be used to get a job done. If you have a job to do, you need to request an implementation that provides the interface. That implementation can reside within another component. To decide which particular implementation you want, you are using a contract ID, which is a text based identifier. The contract ID is a contract on the behaviour of the implementation, accessible using the well defined interface. The XPCOM runtime system knows which class implements which contract ID, and which component provides it.

Even if your code stays completely within one component, and therefore using COM is not a strict requirement, it is very often used anyway. One reason is flexibility. Another is to allow sharing functionality with those portions of the Mozilla logic that are implemented using JavaScript. Mozilla provides a technology called XPConnect that enables communication between interpreted JavaScript and compiled C++ at runtime.

Whenever you request an instance of a COM object at runtime, a class object will be created and you will be given an interface pointer. For several reasons it was decided that these instances are reference counted. One reason is efficiency, since making unnecessary copies of objects should be avoided. Another requirement is that when data objects must be passed between threads, each thread needs to keep a pointer to the same data object in memory. Finally, the same data object might be referenced by multiple components, or stored in multiple lists.

As the lifetime of each reference is different, it is easiest to have each object maintain a reference count, to remember how often it is currently referenced by something else. When you are given a reference to an object (be it from the XPCOM engine directly or from a function call), you have to make sure that you care for reference counting. You must tell the object whether you want to keep a reference to it, or whether you are finished with it, and remove the reference. That way, the object can decide on its own whether it is still needed. When its not needed anymore, it deletes itself from memory.

In order to implement this generic functionality, all classes in Mozilla that implement any interface share the common base class nsISupports, which implements the reference counter and automatic destruction functionality. A similar base class exists in every implementation of COM.

There is a general rule that you must clean up what you allocate. For instance, if you add a reference, you are strongly encouraged to release the reference as soon as it is no longer needed. If you don’t, you might cause problems such as memory leaks.

In C++, this can be done by explicit method calls to methods of the nsISupports base class. But calling these methods is not only easy to forget, but it also makes your code less readable – especially since many functions/methods have multiple exit points (i.e. return statements).

You have to make sure that you release all your referenced objects in each of your exit points. To make this easier, and to not have to repeat many release calls, a general helper class has been provided for dealing with pointers to COM objects, whose name is nsCOMPtr. This is something special to XPCOM and makes coding COM much easier. It simulates a pointer by overriding certain operators. Although there might be some edge cases, the following general rule should be followed for nearly all code: Whenever you’d expect to use a pointer variable “interface*” to an object that implements an interface, use a local “nsCOMPtr<interface>” variable instead. As soon as this pointer goes “out of scope”, its destructor will automatically decrease the reference count, triggering destruction if possible.

In interpreted JavaScript this is easier to code, because of garbage collection. There is some magic that automatically releases the references when possible. However, this magic requires that you don’t introduce cycles. For example, if you have two objects, and each contain a reference to the other, but nobody else keeps a reference to them, this can not be detected. Those objects will live in memory for the rest of program execution.

Exceptions / nsresult

Code execution can fail at runtime. One programming mechanism to deal with failure is to use exceptions. While Mozilla uses Exceptions in its JavaScript code portions, it does not in C++. One of several reasons for this is exceptions haven’t always been portable, so what was done in the past has stuck. Mozilla C++ code uses return codes to simulate exceptions. That means that while you can use try-catch blocks in JavaScript, in C++ you should check the return code whenever you call an interface method. That return code is of type nsresult. For this reason, the logical return type, as defined in the IDL file, is mapped to an additional method parameter in the C++ code.

The nsresult type is intended to transport additional failure reasons. Instead of simply reporting failure or success, an integer type is used instead, to allow for the definition of a lot of different error codes.

There are some general result codes, like NS_OK, which is used to indicate that everything went well and program flow can continue, or NS_ERROR_FAILURE, which is used if something went wrong, but no more details need to be provided as of yet.

In addition to that, each component can request its own range of integers to define error codes that will not overlap with those failure codes used in other areas of an application. Look at mozilla/xpcom/base/nsError.h for more information.

Strings in C++

While many application frameworks or class libraries decided to use just a single string class, the Mozilla developers have decided they need something more powerful. They have implemented a hierarchy of several string classes, which allows the dynamic runtime behaviour to be optimized for different situations. Sometimes you just need a fixed size string; sometimes you need a large string that grows over time. Therefore, for example, not only flat strings, but also segmented string types are available.

An additional requirement is that Mozilla has to be fully multi-language. All strings that deal with information shown to a user are therefore using multi-byte Unicode character strings.

The string types are template-based, with the character type as the variable type, to allow the same logic to be used with regular and Unicode strings.

While that approach of having many string classes means a lot of flexibility, the drawback is that learning Mozilla’s string classes is not trivial.

Graphical User Interface / XUL

Most operating systems define their own way to develop graphical user interfaces, and they are mostly different.

For a cross-platform application like Mozilla it is crucial to have a set of technologies that hide the operating system dependent logic from the application logic.

In the past a lot of C/C++ libraries have been coded that were cross-platform. To my knowledge, none of them are used in Mozilla, yet we have created our own graphics system.

When defining the layout of a GUI (graphical user interface), you can choose to go with either of two possibilities. You could define the absolute positions of each UI (user interface) element that you want to appear. This approach actually has been chosen by a lot of GUI libraries. But it has some drawbacks – you are not very flexible when the layout changes when adding more elements, because you have to rearrange all elements to new positions. You also have to do that graphically, to get immediate feedback which elements overlap, etc. But still, the UI might not look as intended when a different font with different metrics has to be used – this can make a UI unusable.

Mozilla developers wanted to have something very flexible. As Mozilla is cross-platform, it has to be very flexible with regards to fonts.

Mozilla developers have chosen to use an approach where the contents of the UI are designed in a logical manner. We don’t currently use a UI editor. We write a file with instructions on how the UI should look. At runtime, the layout engine decides which fonts are available, and considers all the requests that have been defined in the UI description, and creates the actual UI dynamically. This is similar to what the web browser does when displaying web pages.

The web has gone from a mostly text-based system to a very graphically rich environment that has user interfaces akin to many programs. Therefore, it was only natural for a web browser to use web languages in order to define its user interface. It was decided to use an XML based syntax to write the UI description, which has been called XUL (extensible user-interface language, pronounced ‘zool’).

A XUL file describes what elements the UI consists of, and where elements appear. The XUL language defines attributes that allow the programmer to define the user actions that controls will react to. To define the dynamic behaviour of the application, one can define JavaScript functions that will be called when certain user interface events happen. Within those JavaScript functions, you can either do the required application behaviour directly, or call any other application logic available through COM objects, including logic defined in C++.

In addition to the logical representation of the UI, people also prefer to have a pretty looking UI. To define the detailed characteristics of the UI, one uses CSS files, which define, for example, which images will be used to display certain UI elements. This makes it flexible enough to define additional “looks” for the application, which are referred to as “themes” or “skins”. Mozilla defines currently two themes, classic and modern, which are actively maintained by the Mozilla developers. While there are additional themes, they often exist only for certain versions of Mozilla. It is a lot of work for a theme designer to stay in sync with all the changes that happen to the UI each day.

Update: Most Firefox UI descriptions are now written with XHTML instead of XUL.

Build System and Tree

Nowadays Mozilla is mostly used as a group of many shared libraries, loaded dynamically at runtime as needed. The COM system allows for a development environment, where you often need to re-compile only a subset of the application when you make changes to areas of the source code base. This is a very convenient development environment, but it means a slowdown at runtime. On the other hand, it is possible to create an executable where Mozilla’s internal components are mostly linked statically. Due to the size of the application, this link step takes a lot of time, and makes most sense when preparing a package for distribution.

Each component contains its own directory in the root directory of Mozilla. It might also contain subcomponents within it called modules. There are make files throughout the tree which tell Mozilla how to build.

Most of the platform-specific code is contained in only a few places of the tree. The layer that sits between the operating system and the rest of the Mozilla code consists of the interfaces that are implemented by this code. Only the platform-specific code pertaining to the platform in which the build is occurring is built.

Operating system messages are gathered by the platform-specific code and then sent to the platform-independent code in the same way.

Widgets, with respect to the Mozilla project, are OS-independent widgets drawn using platform-specific rendering techniques.

Within the tree, directories named ‘public’ usually contain interface headers, and directories named ‘src’ usually contain interface implementations and non-interface headers.

Building this program can be daunting for someone not used to a project this large. It can take from 20 minutes on a powerful workstation to a couple hours on an older PC. First, you have to get the source, then build it using the rules contained in While it’s building, it will move all the header files to the dist/include directory so that you don’t have to specify the directory of each header. It will also copy all the XUL, graphics, and JavaScript files (collectively called the chrome) to the chrome directory (a child of the directory containing the Mozilla binary). They are mapped to chrome:// URLs as defined in a file called In release versions of Mozilla, the chrome directory will only contain .jar files.

Building Mozilla is only part of the process. If you want to develop, you will have to maintain the tree using a program called Mercurial. When the build fails, you have to solve conflicts that occur when a merge between a file in your tree and one in the repository fails. Also, when you hack the tree, you have to build parts of the trees you modified. Occasionally, you will have to rebuild the entire tree using a process called depending, so that dependencies between source files can be determined. Also, you will occasionally have to rebuild the tree. Most likely, while you are doing this, you will be maintaining your own changes to the tree and trying to keep it in sync with others’ changes.

Application Startup

Mozilla’s COM system relies on type libraries and an internal registry of available components and interfaces. During application startup, a check is being made whether the registry is still up to date. If changed libraries are detected, the registry is updated. Each component is allowed to do initialization during that registration phase. If changed libraries are detected, they are loaded and their initialization logic is executed. Besides changed libraries, other application components are loaded only as they become required.

Internal Notification System

This section describes an engine that is available inside Mozilla. Most of the time you don’t need it, but if you are required to react to certain events, knowing this system might be helpful. The idea of OOP is, you only do your own job. However, it often happens that component A needs to react when another component triggers an action in B. But component B should not know the details of what needed to happen, because its better to keep parts of the code separate. What is required here is: If other components need to react to B’s actions, B should be extended to send out notifications when its actions are triggered. In addition, B keeps a dynamic list at runtime, where it remembers who wants to be notified. At runtime, when A becomes initialized, A informs B that it wants to become a member of the notification list.

To make this more generic, it has been decided to use a central observer service. When component B triggers an action, it just notifies the observer service, specifying a descriptive name of the event. Components like A subscribe to the observer service giving the names of events they would like to observe. Using that principle, only the observer service has to deal with lists of components watching for events. When the observer receives notification for an event, it passes that notification on to all listening components for that event. Look at nsIObserver for more information.


Mozilla was designed to separate code from human language. Whenever you need to use a text string that needs to be shown to a user, you are not allowed to store that string directly in your JavaScript or C++ file. Instead you define a descriptive identifier for your text, which is used in your C++ or JavaScript file. You use that identifier as a key and call members of the string bundle interface to retrieve the actual text. The text itself is stored in separate files, that only contain text. As Mozilla is modular, there are a lot of these files, each owned by a separate module. With that separation, it is easy to have translators create different language versions of these text files.

When defining the UI, there are two kinds of strings. Some strings are known at the time the application is compiled and packaged, like labels for input fields, or the text that appears within the help system. Other text is assembled dynamically at runtime.

Whenever you define text that does not need to be accessed at runtime, you define it in DTD files. You can refer to that text directly in XUL files.

If you need to work with text at runtime, for example if your text contains a placeholder for a user name that needs to be filled at runtime, you define your text in properties files.

Coding and Review Rules

You are free to download Mozilla, change the code, and work with a version that contains your own changes.

However, one idea behind open source as used in Mozilla is the following: You received the source for free, and if you make changes, you should consider giving something back to the community. By doing so, you become a contributor.

But the Mozilla community has decided that it can’t accept just any change to be integrated into the public central Mozilla codebase. If you want your code to become a part of it, you need to follow rules. These rules are not like law, but basically you must convince people that your change is good.

The idea is, a lot of effort has been made to get the Mozilla code to its current state. While the Mozilla code is, like every piece of software, far from being perfect, people try to avoid anything that decreases maintainability, and to only allow code that seems to be correct.

In order to achieve this, the Mozilla community has decided that all code has to be reviewed by other already known Mozilla hackers. There are two levels of review. You should get a first review (r=) from the owner of the code area you are changing. You are expected to make the corrections that are requested, or your code will not go in. If you have that first review, you need to get what is called super-review (sr=) in most cases. There are only a limited number of “Mozilla gurus” that have an accepted reputation for making good decisions on what code is good and what needs to be changed. Once you have both reviews, your code may be checked in most of the time. There are other levels of review in certain instances, and that is described elsewhere.

Many people make changes to Mozilla. While everybody tries to make Mozilla better, every change has the risk of unforeseen side effects. These range from application features that, as a result of the change, no longer work, to the problem that the Mozilla source code simply does not compile anymore. The latter is called “the tree is broken, burning, or red”, where tree refers to the Mercurial source repository.

If you are developing only on one combination of operating system and compiler, and you are not paying attention to the portability rules (read the documents on, it is very easy to break the tree. You should try your best to not break it, and getting review hopefully will find potential problems early, before the changes are checked in.

A broken tree sucks. The Mozilla community has decided on the rule, that no other check-ins are allowed while the tree is broken. This is to help people who try to fix the problem. Allowing additional changes would make it even more difficult to find the real cause of a problem, because any new check-in could contain new problems.


Every several weeks produces a new snapshot of the source repository. The idea is, people in the world should try out the current snapshot, and report any problems (bugs) they find. However, wants to emphasize that these milestones are being produced for testing purposes only.

In order to prepare a more stable milestone, the rules for making changes to the source code repository get more restrictive prior to producing a new milestone. defines a schedule, and a couple of days before a milestone date, only those check-ins are allowed that have been approved by the “drivers”. Drivers are a bunch of people who have the highest authority regarding the Mozilla repository. The drivers will also decide whether the milestone seems ready, or whether the schedule will be delayed in order to allow more fixes to be produced, that will make it more stable.


Bugzilla is’s web based bug tracking system. Whenever a problem is encountered, people are asked to file a new bug (sometimes also known as problem ticket), with a good description of what happened. You will then be issued a bug or ticket number. This number will be used when people talk about the problem. Developers will subscribe to the “bug” and add comments to it, and if they know a fix, they will attach a patch to show how they suggest to fix the problem. Reviews will also be tracked within this system.

The term “bug” is most often referred to as a software error. However, within Bugzilla, a bug can be anything that needs to be tracked. This ranges from software errors to enhancement requests. For example, the evolution of this document is tracked in bug #123230.

If you are not a C++ developer, you can also work on Bugzilla. It started out as a simple bug reporting tool and has turned into quite a functional, complex system that has many users from other projects (such as Redhat).

Webtools / DXR / Bonsai

Webtools are server based tools that store information, and allow their information to be displayed and maybe even manipulated. You access such systems using a web browser, e.g. Mozilla.

Mozilla developers have created several tools in order to make their life easier. We have already talked about Bugzilla.

DXR (earlier called MXR, and LXR before that) is a hypertext search engine for the Mozilla source code. You can look up identifiers and text, and you will see where in Mozilla it is used. Clicking on search result entries will directly bring you to current source code. The code is shown in a web page, and identifiers are hyperlinks, which, when clicked, will bring you the DXR search result for that identifier. You can use DXR to browse the source code and quickly navigate through it. It is based on the Linux project’s glimpse engine with internal modifications.

Tinderbox is a tool that shows the current state of the source repository. hosts several machines for many different operating systems, that repeatedly and constantly obtain (check out) the newest source code from the central repository, and try to compile it. When finished with compiling, some automated tests are executed to check, whether the program still works correctly. The results of compilation and tests are reported to the central tinderbox system. When you access the tinderbox page, you can see whether the source code repository is currently in good condition, and how things changed in the past few hours. Tinderbox shows a graph, where time is shown vertically and the different machines / operating systems are displayed horizontally. Each compilation test phase is represented by a bar, defined by the required time for building.

The bar is filled with a color. Green means good. Yellow means, compilation is still in progress, orange means compilation and building were successful, but one of the automated tests failed. Red means, the source code did not even compile. If the tree is red, it slows down development.

Tinderbox is a very helpful tool, and everybody making changes to the source repository is expected to “watch the tree”, i.e. watch whether your changes cause anything to fail.

To assist you more, additional information is available on the tinderbox page. As people check in, their names appear on the page. There is a link to a list of changes they have made. If a compilation or test failed, the box will contain links to the output from the compiler, which should show the reason for failure. Additional text on the page shows the results of performance measurement.

Another web tool is Bonsai. Bonsai tracks all changes made to the source repository. You can retrieve a listing of all changes made by people. It also provides a powerful interface to query the list of changes.

Finding more information

If you want to learn more about the mentioned programming techniques in general, I recommend searching the web for free documents. Hopefully the mentioned terms in this document will guide you. If you prefer to read books, I suggest to choose those books in which the author tries to explain in general, and does not concentrate on some particular operating system.

Most documentation regarding Mozilla development is published on the web site. If you can’t find what you are looking for, try to use a search engine. Some popular search engines offer an advanced search option, where one can limit the search to a specific web site.