We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.

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 instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.

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,