Firefox addons developer guide




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.


The Firefox add-ons developer guide was contributed by the Mozilla Japan community; it covers how to go about building Firefox extensions using XPCOM and XUL. These days, we recommend using the Add-on SDK instead, but there are times when you need the additional control offered by a more direct approach.



  1. Introduction to extensions
  2. Technologies used in developing extensions
  3. Introduction to XUL—How to build a more intuitive UI
  4. Using XPCOM—Implementing advanced processes
  5. Let's build a Firefox extension
  6. Firefox extensions and XUL applications