This is an archived page.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.



Listen to changes to the Preferences system in Firefox. This enables add-ons to listen to change events to the system-wide settings. These are the same preferences that are exposed to users in the about:config page.
preferences/service gives you full access to the preferences system. You can also use the simple-prefs module to access just the preferences for your own add-on and expose them to the user in the Add-on Manager.





options : object
Required options:

Name Type  
branchName string

By default this is "", the root.


var { PrefsTarget } = require("sdk/preferences/event-target");

// listen to the same branch which reqire("sdk/simple-prefs") does
var target = PrefsTarget({ branchName: "extensions." + require("sdk/self").preferencesBranch + "." });

target.once("test", function(prefName) {
  console.log(prefName) // logs "test"
  console.log(target.prefs[name]) // logs true

target.once("", function() {
  console.log(prefName) // logs "test"
  console.log(target.prefs[name]) // logs true

// changing a pref which our target listens to
require("sdk/simple-prefs").prefs.test = true;