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 an API for creating namespaces for objects, which effectively may be used for creating fields that are not part of objects public API.

  let { ns } = require('sdk/core/namespace');
  let aNamespace = ns();

  aNamespace(publicAPI).secret = secret;

One namespace may be used with multiple objects:

  let { ns } = require('sdk/core/namespace');
  let dom = ns();

  function View(element) {
    let view = Object.create(View.prototype);
    dom(view).element = element;
    // ....
  View.prototype.destroy = function destroy() {
    let { element } = dom(this);
    // ...
  // ...
  exports.View = View;
  // ...

Also, multiple namespaces can be used with one object:

  // ./widget.js

  let { Cu } = require('chrome');
  let { ns } = require('sdk/core/namespace');
  let { View } = require('./view');

  // Note this is completely independent from View's internal Namespace object.
  let sandboxes = ns();

  function Widget(options) {
    let { element, contentScript } = options;
    let widget = Object.create(Widget.prototype);, options.element);
    sandboxes(widget).sandbox = Cu.Sandbox(element.ownerDocument.defaultView);
    // ...
  Widget.prototype = Object.create(View.prototype);
  Widget.prototype.postMessage = function postMessage(message) {
    let { sandbox } = sandboxes(this);
  Widget.prototype.destroy = function destroy() {;
    // ...
    delete sandboxes(this).sandbox;
  exports.Widget = Widget;

In addition access to the namespace can be shared with other code by just handing them a namespace accessor function.

  let { dom } = require('./view');
  Widget.prototype.setInnerHTML = function setInnerHTML(html) {
    dom(this).element.innerHTML = String(html);

Document Tags and Contributors

Contributors to this page: wbamberg
Last updated by: wbamberg,