This is an archived page.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


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.


Provides functions for communicating with an embedded WebExtension.

This API is supported from Firefox 51 onwards, and is only available if you have set the "hasEmbeddedWebExtension" key in package.json.




This function starts the embedded WebExtension.


Promise : a promise that resolves to an object with a single property browser.

The browser object contains the runtime APIs that the embedding add-on can use to exchange messages with the embedded WebExtension:

For example:

// SDK add-on

const webExtension = require("sdk/webextension");

webExtension.startup().then(api => {
  const {browser} = api;

See the page on embedded WebExtensions for all the details on embedding WebExtensions in SDK add-ons.