Add-ons allow developers to extend and modify the functionality of Firefox. They are written using standard Web technologies - JavaScript, HTML, and CSS - plus some dedicated JavaScript APIs.

Among other things, an add-on could:

  • Change the appearance or content of particular websites
  • Modify the Firefox user interface
  • Add new features to Firefox

There are several types of add-ons, but the most common type are extensions.

Developing extensions

In the past, there were several toolsets for developing Firefox extensions, but as of November 2017, extensions must be built using WebExtensions APIs. Other toolsets, such as overlay add-ons, bootstrapped add-ons, and the Add-on SDK, are no longer supported.

Extensions written using WebExtensions APIs for Firefox are designed to be cross-browser compatible. In most cases, it will run in Chrome, Edge, and Opera with few if any changes. They are also fully compatible with multiprocess Firefox. You can see the APIs currently supported in Firefox and other browsers.

Extension Workshop

The Extension Workshop can help you develop extensions for Firefox and give your users simple, yet powerful ways to customize their browsing experience. You’ll find:

Extensions for Firefox for Android

In 2020, Mozilla will release a new Firefox for Android experience. This new, high-performance browser for Android has been rebuilt from the ground up using GeckoView, Mozilla’s mobile browser engine. We are currently building support for the WebExtensions API on GeckoView.

Migrate an existing extension

If you maintain a legacy extension, such as an XUL overlay, bootstrapped, or Add-on SDK-based extension, you can still port it to use WebExtension APIs.  There are some porting resources on Extension Workshop, our site for Firefox-specific development resources.

For more information about transition support, please visit our wiki page.

Publishing add-ons, commonly known as "AMO," is Mozilla's official site for developers to list add-ons, and for users to discover them. By uploading your add-on to AMO, you can participate in our community of users and creators and find an audience for your add-on.

You are not required to list your add-on on AMO, but your add-on must be signed by Mozilla else users will not be able to install it.

For an overview for the process of publishing your add-on see Signing and distributing your add-on.

Other types of add-ons

In addition to extensions, there are a few other add-on types that allow users to customize Firefox. Those add-ons include:

  • User dictionaries let you spell-check in different languages.

  • Language packs let you have more languages available for the user interface of Firefox. 

Contact us

You can use the links below to get help, keep up to date with news around add-ons and give us feedback.

Add-ons forum

Use the Add-ons Discourse forum to discuss all aspects of add-on development and to get help.

Mailing lists

Use the dev-addons list to discuss development of the add-ons ecosystem, including the development of the WebExtensions system and of AMO:


Matrix is an open, lightweight protocol for decentralized, real-time communications. See this wiki page for information about how to join Mozilla's Matrix instance.

  • Add-ons (support for extensions, themes, and the WebExtensions API)
  • AMO (discussion around

Report problems

Security vulnerabilities

If you discover a security vulnerability in an add-on, even if it is not hosted on a Mozilla site, let us know and we will work with the developer to correct the issue. Please report them confidentially in Bugzilla or by emailing

Bugs on (AMO)

If you find a problem with the site, we'd love to fix it. Please file a bug report and include as much detail as possible.