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.


The BrowserApp object is only available to privileged code running on Firefox for Android, and is intended for use by Firefox for Android add-ons.


deck is a property of the BrowserApp object. It is a deck of all currently open browsers, represented as browser objects.

Tab events

Using the deck property, you can listen to three tab events: TabOpen, TabClose, and TabSelect. For all three events, the event's target is the browser hosted by that tab.

Here's an add-on that uses the template "bootstrap.js" introduced in the Initialization and Cleanup guide and implements loadIntoWindow()to listen to all three of those tab events. Note that it also implements unloadFromWindow() to remove the listeners.

var gWindow;

function logTabOpen(event) {
  gWindow.console.log("Log_tabs: Opening new tab");

function logTabClose(event) {
  let browser =;  
  gWindow.console.log("Log_tabs: Closing: " + browser.currentURI.spec);

function logTabSelect(event) {
  let browser =;  
  gWindow.console.log("Log_tabs: Selecting: " + browser.currentURI.spec);

function loadIntoWindow(window) {
  if (!window)
  gWindow = window;
  gWindow.console.log("Log_tabs: starting");
  window.BrowserApp.deck.addEventListener("TabOpen", logTabOpen, false);
  window.BrowserApp.deck.addEventListener("TabClose", logTabClose, false);
  window.BrowserApp.deck.addEventListener("TabSelect", logTabSelect, false);

function unloadFromWindow(window) {
  if (!window)
  window.BrowserApp.deck.removeEventListener("TabOpen", logTabOpen, false);
  window.BrowserApp.deck.removeEventListener("TabClose", logTabClose, false);
  window.BrowserApp.deck.removeEventListener("TabSelect", logTabSelect, false);

Document Tags and Contributors

Contributors to this page: rebloor, mdnwebdocs-bot, wbamberg, andrewtruongmoz
Last updated by: rebloor,