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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 addons.mozilla.org (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.

There are a couple of differences between desktop Firefox and Firefox for Android that are particularly relevant to add-on developers:

  • there is no visible XUL in the user interface, so you can't use XUL overlays to create your add-on's UI
  • the gBrowser object does not exist, so you can't use tabbrowser to interact with browser tabs

Instead, Firefox for Android provides its own APIs:

In these pages we've documented the main functions and properties exposed by these objects. To see all the details, refer to the code at browser.js.

We also have some modules that don't ship with Firefox for Android but can be used in addons:

  • Sound.jsm lets you play sounds in the browser simply

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 Last updated by: rebloor,