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.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
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 WebExtensions if they can. See the "Compatibility Milestones" document for more.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
XUL School is a comprehensive add-on development tutorial, focusing on Firefox extension development. It is recommended that you read through all of it at least once. While Firefox changes rapidly, the content in this tutorial should be up to date and valid.
- Basic functionality
- Intermediate functionality
- Advanced topics
The XUL School project was developed by Appcoast (formerly Glaxstar). The project is now published here following its sharing licenses. Its contents have been modified from the original source as necessary.