We are planning to deprecate the use, in Firefox, of the techniques described in this document.

Don't use these techniques to develop new add-ons. Use WebExtensions or the Add-on SDK instead.

If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions or the SDK instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions or the Add-on SDK.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

This page contains links to documentation for the approach to developing extensions for Gecko-based applications which uses:

  • XUL overlays to specify the interface
  • APIs available to privileged code, such as tabbrowser and JavaScript modules, to interact with the application and content.

Prior to Firefox 4, and the Gecko 2 engine that powers it, this was the only way to develop extensions. This methodology has largely been superseded by restartless extensions, and the Add-on SDK, which is built on top of them. The privileged JavaScript APIs described here can still be used in these newer types of add-ons.

XUL School

XUL School is a comprehensive add-on development tutorial, focusing on Firefox extension development but mostly applicable to other Gecko-based applications.

More resources

Setting up your environment
Setting up the application for extension development.
Tutorials and reference for the user interface language used by XUL extensions.
Code snippets
Sample code for many of the things you'll want to do.
Installing extensions
How to install an extension by copying the extension files into the application's install directory.
Firefox add-ons developer guide
A guide to developing overlay extensions.
JavaScript code modules
JavaScript modules available to extension developers.
Extension preferences
How to specify the preferences for your extension that will appear in the Add-ons Manager.
Extension packaging
How extensions are packaged and installed.
Binary Firefox extensions
Creating binary extensions for Firefox.


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 Last updated by: wbamberg,