Note: Do not make substantive changes to this document without consulting Benjamin Smedberg <email@example.com> or one of the build-config peersThis document details how to configure Firefox builds. Most of the time a
mozconfig file is not required. The default options are the most well-supported, so it is preferable to add as few options as possible. Please read the following directions carefully before building, and follow them in order. Skipping any step may cause the build to fail, or the built software to be unusable. Build options, including options not usable from the command-line, may appear in "
confvars.sh" files in the source tree.
mozconfig configuration file
The choice of which Mozilla application to build and other configuration options can be configured in a
mozconfig file. (It is possible to manually call
configure with command-line options, but this is not recommended). The
mozconfig file should be in your source directory (that is,
Create a blank
echo "# My first mozilla config" > mozconfig
If your mozconfig isn't in your source directory, you can also use the
MOZCONFIG environment variable to specify the path to your
mozconfig. The path you specify must be an absolute path or else
client.mk will not find it. This is useful if you choose to have multiple
mozconfig files for different applications or configurations (see below for a full example). Note that in the
export example below the filename was not
mozconfig. Regardless of the name of the actual file you use, we refer to this file as the
mozconfig file in the examples below.
Calling the file
.mozconfig (with a leading dot) is also supported, but this is not recommended because it may make the file harder to find. This will also help when troubleshooting because people will want to know which build options you have selected and will assume that you have put them in your
mozconfig contains two types of options:
- Options prefixed with
mk_add_optionsare passed to
client.mk. The most important of these is
MOZ_OBJDIR, which controls where your application gets built (also known as the object directory).
- Options prefixed with
ac_add_optionsare passed to
configure, and affect the build process.
Building with an objdir
This means that the source code and object files are not intermingled in your directory system and you can build multiple applications (e.g., Firefox and Thunderbird) from the same source tree. If you do not specify a
MOZ_OBJDIR, it will be automatically set to
If you need to re-run
configure, the easiest way to do it is using
./mach configure; running
configure manually is strongly discouraged.
Adding the following line to your
mozconfig allows you to change the objdir:
It is a good idea to have your objdir name start with
obj so that Mercurial ignores it.
Sometimes it can be useful to build multiple versions of the source (such as with and without diagnostic asserts). To avoid the time it takes to do a full rebuild, you can create multiple
mozconfig files which specify different objdirs. For example, a
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-ff-dbg ac_add_options --enable-debug
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-ff-rel-opt ac_add_options --disable-debug ac_add_options --enable-optimize
allow for building both versions by specifiying the configuration via the
MOZCONFIG environment variable:
$ env MOZCONFIG=/path/to/mozconfig-dbg ./mach build $ env MOZCONFIG=/path/to/mozconfig-rel-opt ./mach build
Don't forget to set the
MOZCONFIG environment variable for the
mach run command as well.
Be aware that changing your
mozconfig will require the configure process to be rerun and therefore the build will take considerably longer, so if you find yourself changing the same options regularly, it may be worth having a separate
mozconfig for each. The main downside of this is that each objdir will take up a significant amount of space on disk.
Most modern systems have multiple cores or CPUs, and they can be optionally used concurrently to make the build faster. The
-j flag controls how many parallel builds will run concurrently. You will see (diminishing) returns up to a value approximately 1.5× to 2.0× the number of cores on your system.
If your machine is overheating, you might want to try a lower value, e.g.
Choose an application
--enable-application=application flag is used to select an application to build. Firefox is the default.
Choose one of the following options to add to your
- Browser (Firefox)
Note: This is the default
- Mail (Thunderbird)
- Mozilla Suite (SeaMonkey)
- Calendar (Lightning Extension, uses Thunderbird)
ac_add_options --enable-application=comm/mail ac_add_options --enable-calendar
Selecting build options
The build options you choose depends on what application you are building and what you will be using the build for. If you want to use the build regularly, you will want a release build without extra debugging information; if you are a developer who wants to hack the source code, you probably want a non-optimized build with extra debugging macros.
There are many options recognized by the configure script which are special-purpose options intended for embedders or other special situations, and should not be used to build the full suite/XUL applications. The full list of options can be obtained by running
Warning: Do not use a configure option unless you know what it does. The default values are usually the right ones. Each additional option you add to your
mozconfig file reduces the chance that your build will compile and run correctly.
The following build options are very common:
Enables the default compiler optimization options
Note: This is enabled by default
- Chooses particular compiler optimization options. In most cases, this will not give the desired results, unless you know the Mozilla codebase very well; note, however, that if you are building with the Microsoft compilers, you probably do want this as
-O1will optimize for size, unlike GCC.
- Disables compiler optimization. This makes it much easier to step through code in a debugger.
You can make an optimized build with debugging symbols. See Building Firefox with Debug Symbols.
- There are many optional pieces of code that live in
extensions/. Many of these extensions are now considered an integral part of the browsing experience. There is a default list of extensions for the suite, and each app-specific
mozconfigspecifies a different default set. Some extensions are not compatible with all apps, for example:
cookieis not compatible with thunderbird
typeaheadfindis not compatible with any toolkit app (Firefox, Thunderbird)
Unless you know which extensions are compatible with which apps, do not use the
--enable-extensionsoption; the build system will automatically select the proper default set of extensions.
- By default, many auxiliary test applications are built, which can help debug and patch the mozilla source. Disabling these tests can speed build time and reduce disk space considerably. Developers should generally not use this option.
- If a clobber would be required before a build, this will cause mach to clobber and continue with the build instead of asking the user to manually clobber and exiting.
- Cryptography is enabled by default. In some countries, it may be illegal to use or export cryptographic software. You should become familiar with the cryptography laws in your country.
On the 1.7 and aviary branches, cryptography was off by default. You need to specify
--enable-cryptoif you want SSL, S/MIME, or other software features that require cryptography.
- This enables the machinery that allows Firefox to write out a minidump files when crashing as well as the tools to process and submit crash reports to Mozilla. After enabling the crash reporter in your local build, you will need to run mach with the --enable-crash-reporter (note the extra dash) to enable it at runtime, like so: `
./mach run --enable-crash-reporter`
- This makes compiler warnings into errors which fail the build. This can be useful since certain warnings coincide with reviewbot lints which must be fixed before merging.
- This enables sccache to speed up builds. This saves a considerable amount of time directly after a clobber build, so pulling in new updates and rebasing is less time-consuming.
Mozilla's official builds use mozconfig files from the appropriate directory within each repository.
mozconfig files are taken from production builds and are provided as examples only. It is recommended to use the default build options, and only change the properties from the list above as needed. The production builds aren't really appropriate for local builds.
Building multiple applications from the same source tree
It is possible to build multiple applications from the same source tree, as long as you use a different objdir for each application.
You can either create multiple
mozconfig files, or alternatively, use the
MOZ_BUILD_PROJECTS make option.
MOZ_BUILD_PROJECTS in a single
MOZ_BUILD_PROJECTS, you must specify a
MOZ_OBJDIR and a
MOZ_BUILD_PROJECTS make option, containing space separated names. Each name can be an arbitrary directory name. For each name, a subdirectory is created under the toplevel objdir. You then need to use the
ac_add_app_options with the specified names to enable different applications in each object directory.
ac_add_options --disable-optimize --enable-debug mk_add_options MOZ_OBJDIR=/mozilla/src/obj-@CONFIG_GUESS@ mk_add_options MOZ_BUILD_PROJECTS="browser mail" ac_add_app_options browser --enable-application=browser ac_add_app_options mail --enable-application=comm/mail
If you want to build only one project using this
mozconfig, use the following command line:
MOZ_CURRENT_PROJECT=browser ./mach build
This will build only the browser.
Using multiple mozconfig files
Alternatively, you may want to create separate
As an example, the following steps can be used to build Firefox and Thunderbird. You should first create three
# add common options here, such as making an optimized release build mk_add_options MOZ_MAKE_FLAGS="-j4" ac_add_options --enable-optimize --disable-debug
# include the common mozconfig . ./mozconfig-common # Build Firefox mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-firefox ac_add_options --enable-application=browser
# include the common mozconfig . ./mozconfig-common # Build Thunderbird mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-thunderbird ac_add_options --enable-application=comm/mail
To build Firefox, run the following commands:
export MOZCONFIG=/path/to/mozilla/mozconfig-firefox ./mach build
To build Thunderbird, run the following commands:
export MOZCONFIG=/path/to/mozilla/mozconfig-thunderbird ./mach build
Mozconfigwrapper is similar to using multiple mozconfig files except that it abstracts and hides them so you don't have to worry about where they live or which ones you've created. It also saves you from having to export the MOZCONFIG variable each time. For information on installing and configuring mozconfigwrapper, see https://github.com/ahal/mozconfigwrapper.