Using XPCOM without chrome


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.



Using sdk XPCOM

With the low-level module sdk/platform/xpcom , it's possible to exclude chrome and XPCOMUtils in some cases.
Below is an example for the XPCOM bookmark observer.


Bookmarks Observer

Normally, a bookmark observer would require Chrome Components and XPCOMUtils as described in the following links:

However, with the sdk module , we can remove this need.
Below is an example, where we extend the xpcom module's Unknown class with an nsINavBookmarkObserverinterface and one of its optional interface methods (onItemchanged). We can then create an object from it and add it as an observer to the bookmarks service.

// This removes the need to import Ci and the XPCOMUtils
const { Class }       = require("sdk/core/heritage");
const { Unknown }     = require('sdk/platform/xpcom');
const { PlacesUtils } = require("resource://gre/modules/PlacesUtils.jsm");

let bmListener = Class({
 extends: Unknown,
 interfaces: [ "nsINavBookmarkObserver" ],
 //This event most often handles all events
 onItemChanged: function(bId, prop, an, nV, lM, type, parentId, aGUID, aParentGUID) {
  console.log("onItemChanged", "bId: "+bId, "property: "+prop, "isAnno: "+an, "new value: "+nV, "lastMod: "+lM, "type: "+type, "parentId:"+parentId, "aGUID:"+aGUID);0

  // Code to handle the event here


//We just have a class, but need an object. Notice the small l
var bmlistener = bmListener();
PlacesUtils.bookmarks.addObserver(bmlistener, false);