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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 one of the building blocks for those modules that use content scripts to interact with web content, such as panel and page-mod.


The module exports a constructor for the Loader object, which inherits on(), once(), and removeListener() functions that enable its users to listen to events.

Loader adds code to initialize and validate a set of properties for managing content scripts:

  • contentURL
  • contentScript
  • contentScriptFile
  • contentScriptWhen
  • contentScriptOptions
  • allow

When certain of these properties are set, the Loader emits a propertyChange event, enabling its users to take the appropriate action.

The Loader is used by modules that use content scripts but don't themselves load content, such as page-mod.

Modules that load their own content, such as panel and page-worker, use the symbiont module instead. Symbiont inherits from Loader but contains its own frame into which it loads content supplied as the contentURL option.


The following code creates a wrapper on a hidden frame that reloads a web page in the frame every time the contentURL property is changed:

var hiddenFrames = require("sdk/frame/hidden-frame");
var { Loader } = require("sdk/content/content");
var PageLoader = Loader.compose({
  constructor: function PageLoader(options) {
    options = options || {};
    if (options.contentURL)
      this.contentURL = options.contentURL;
    this.on('propertyChange', this._onChange = this._onChange.bind(this));
    let self = this;
      onReady: function onReady() {
        let frame = self._frame = this.element;
        self._emit('propertyChange', { contentURL: self.contentURL });
  _onChange: function _onChange(e) {
    if ('contentURL' in e)
      this._frame.setAttribute('src', this._contentURL);




The local file URLs of content scripts to load. Content scripts specified by this property are loaded before those specified by the contentScript property.


The texts of content scripts to load. Content scripts specified by this property are loaded after those specified by the contentScriptFile property.


When to load the content scripts. This may take one of the following values:

  • "start": load content scripts immediately after the document element for the page is inserted into the DOM, but before the DOM content itself has been loaded
  • "ready": load content scripts once DOM content has been loaded, corresponding to the DOMContentLoaded event
  • "end": load content scripts once all the content (DOM, JS, CSS, images) for the page has been loaded, at the time the window.onload event fires


Read-only value exposed to content scripts under self.options property.

Any kind of jsonable value (object, array, string, etc.) can be used here. Optional.


The URL of the content loaded.


Permissions for the content, with the following keys: