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 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.

Getting the directory where your add-on is located

If you need to determine the directory in which your add-on is installed, code like the following will do the trick. Simply replace YOUREXTENSIONID with your add-on's ID.

Components.utils.import("resource://gre/modules/AddonManager.jsm");

AddonManager.getAddonByID("YOUREXTENSIONID", function(addon) {
  var addonLocation = addon.getResourceURI("").QueryInterface(Components.interfaces.nsIFileURL).file.path;
});

Accessing file and version information

Components.utils.import("resource://gre/modules/AddonManager.jsm");

AddonManager.getAddonByID("my-addon@foo.com", function(addon) {
  alert("My extension's version is " + addon.version);
  alert("Did I remember to include that file.txt file in my XPI? " +
        addon.hasResource("file.txt") ? "YES!" : "No");
  alert("Let's pretend I did, it's available from the URL " + addon.getResourceURI("file.txt").spec);
});

Uninstall an add-on

Components.utils.import("resource://gre/modules/AddonManager.jsm");

AddonManager.getAddonByID("youraddon@youraddon.com", function(addon) {
  addon.uninstall();
});

Disable an add-on

Components.utils.import("resource://gre/modules/AddonManager.jsm");

AddonManager.getAddonByID("youraddon@youraddon.com", function(addon) {
  if (addon.isActive) addon.userDisabled = addon.isActive;
});

Listening for add-on uninstall

This example sets a variable beingUninstalled that you can check when you get a profile-before-change message to do cleanup for your add-on on uninstall.

var beingUninstalled;

let listener = {
  onUninstalling: function(addon) {
    if (addon.id == "youraddon@youraddon.com") {
      beingUninstalled = true;
    }
  },
  onOperationCancelled: function(addon) {
    if (addon.id == "youraddon@youraddon.com") {
      beingUninstalled = (addon.pendingOperations & AddonManager.PENDING_UNINSTALL) != 0;
    }
  }
}

try {
  Components.utils.import("resource://gre/modules/AddonManager.jsm");

  AddonManager.addAddonListener(listener);
} catch (ex) {}