This is an archived page. It's not actively maintained.

Overlay extensions


Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

Starting from Firefox 53, no new legacy add-ons will be accepted on (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

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 This methodology has largely been superseded by restartless extensions, and the Add-on SDK, which is built on top of them.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.