MDN wants to learn about developers like you:

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 Add-on SDK supports event-driven programming.

Objects emit events on state changes that might be of interest to add-on code, such as browser windows opening, pages loading, network requests completing, and mouse clicks. By registering a listener function to an event emitter an add-on can receive notifications of these events.

We talk about content scripts in more detail in the Working with Content Scripts guide.

Additionally, if you're using content scripts to interact with web content, you can define your own events and use them to communicate between the main add-on code and the content scripts. In this case one end of the conversation emits the events, and the other end listens to them.

So there are two main ways you will interact with the EventEmitter framework:

  • listening to built-in events emitted by objects in the SDK, such as tabs opening, pages loading, mouse clicks

  • sending and receiving user-defined events between content scripts and add-on code

This guide only covers the first of these; the second is explained in the Working with Content Scripts guide.

Adding Listeners

You can add a listener to an event emitter by calling its on(type, listener) method.

It takes two parameters:

  • type: the type of event we are interested in, identified by a string. Many event emitters may emit more than one type of event: for example, a browser window might emit both open and close events. The list of valid event types is specific to an event emitter and is included with its documentation.

  • listener: the listener itself. This is a function which will be called whenever the event occurs. The arguments that will be passed to the listener are specific to an event type and are documented with the event emitter.

For example, the following add-on registers a listener with the tabs module to listen for the ready event, and logs a string to the console reporting the event:

var tabs = require("sdk/tabs");

tabs.on("ready", function () {
  console.log("tab loaded");

It is not possible to enumerate the set of listeners for a given event.

The value of this in the listener function is the object that emitted the event.

Listening to all events

You can pass the wildcard "*" as the type argument. If you do this, the listener will be called for any event emitted by that object, and its argument will be the name of the event:

var ui = require("sdk/ui");
var panels = require("sdk/panel");
var self = require("sdk/self");

var panel = panels.Panel({

panel.on("*", function(e) {
  console.log("event " + e + " was emitted");

var button = ui.ActionButton({
  id: "my-button",
  label: "my button",
  icon: "./icon-16.png",
  onClick: handleClick

function handleClick(state) {{
    position: button

This wildcard feature does not yet work for the tabs or windows modules.

Adding Listeners in Constructors

Event emitters may be modules, as is the case for the ready event above, or they may be objects returned by constructors.

In the latter case the options object passed to the constructor typically defines properties whose names are the names of supported event types prefixed with "on": for example, "onOpen", "onReady" and so on. Then in the constructor you can assign a listener function to this property as an alternative to calling the object's on() method.

For example: the ActionButton object emits an event when the button is clicked.

The following add-on creates a button and assigns a listener to the onClick property of the options object supplied to the button's constructor. The listener loads

  id: "visit-mozilla",
  label: "Visit Mozilla",
  icon: "./icon-16.png",
  onClick: function() {

This is exactly equivalent to constructing the button and then calling the button's on() method:

var button = require("sdk/ui/button/action").ActionButton({
  id: "visit-mozilla",
  label: "Visit Mozilla",
  icon: "./icon-16.png"

button.on("click", function() {

Removing Event Listeners

Event listeners can be removed by calling removeListener(type, listener), supplying the type of event and the listener to remove.

The listener must have been previously been added using one of the methods described above.

In the following add-on, we add two listeners to the tabs module's ready event. One of the handler functions removes the listener again.

Then we open two tabs.

var tabs = require("sdk/tabs");

function listener1() {
  console.log("Listener 1");
  tabs.removeListener("ready", listener1);

function listener2() {
  console.log("Listener 2");

tabs.on("ready", listener1);
tabs.on("ready", listener2);"");"");

We should see output like this:

info: tabevents: Listener 1
info: tabevents: Listener 2
info: tabevents: Listener 2

Listeners will be removed automatically when the add-on is unloaded.

Document Tags and Contributors

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