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JavaScript APIs for WebExtensions can be used inside the extension's background scripts and in any other documents bundled with the extension, including browser action or page action popups, sidebars, options pages, or new tab pages. A few of these APIs can also be accessed by an extension's content scripts (see the list in the content script guide).

To use the more powerful APIs you need to request permission in your extension's manifest.json.

You can access the APIs using the browser namespace:

function logTabs(tabs) {

browser.tabs.query({currentWindow: true}, logTabs);

Many of the APIs are asynchronous, returning a Promise:

function logCookie(c) {

function logError(e) {

var setCookie = browser.cookies.set(
  {url: "https://developer.mozilla.org/"}
setCookie.then(logCookie, logError);

Note that this is different from Google Chrome's extension system, which uses the chrome namespace instead of browser, and which uses callbacks instead of promises for asynchronous functions. As a porting aid, the Firefox implementation of WebExtensions APIs supports chrome and callbacks as well as browser and promises. Mozilla has also written a polyfill which enables code that uses browser and promises to work unchanged in Chrome: https://github.com/mozilla/webextension-polyfill.

Firefox also implements these APIs under the chrome namespace using callbacks. This allows code written for Chrome to run largely unchanged in Firefox for the APIs documented here.

Microsoft Edge uses the browser namespace, but doesn't yet support promise-based asynchronous APIs. In Edge, for the time being, asynchronous APIs must use callbacks.

Not all browsers support all the APIs: for the details, see Browser support for JavaScript APIs.


Schedule code to run at a specific time in the future. This is like setTimeout() and setInterval(), except that those functions don't work with background pages that are loaded on demand.

API reference documentation


The WebExtensions {{WebExtAPIRef("bookmarks")}} API lets an extension interact with and manipulate the browser's bookmarking system. You can use it to bookmark pages, retrieve existing bookmarks, and edit, remove, and organize bookmarks.

API reference documentation


Adds a button to the browser's toolbar.

API reference documentation


API reference documentation


Enables extensions to clear the data that is accumulated while the user is browsing.

API reference documentation


The clipboard API enables an extension to copy items to the system clipboard. Currently the API only supports copying images, but it's intended to support copying text and HTML in the future.

API reference documentation


Listen for the user executing commands that you have registered using the commands manifest.json key.

API reference documentation


Use this API to register content scripts. Registering a content script instructs the browser to insert the given content scripts into pages that match the given URL patterns.

API reference documentation


Work with contextual identities: list, create, remove, and update contextual identities.

API reference documentation


Enables extensions to get and set cookies, and be notified when they change.

API reference documentation


The devtools.inspectedWindow API lets a devtools extension interact with the window that the developer tools are attached to.

API reference documentation


The devtools.network API lets a devtools extension get information about network requests associated with the window that the devtools are attached to (the inspected window).

API reference documentation


The devtools.panels API lets a devtools extension define its user interface inside the devtools window.

API reference documentation


Enables extensions to interact with the browser's download manager. You can use this API module to download files, cancel, pause, resume downloads, and show downloaded files in the file manager.

API reference documentation


Common types used by APIs that dispatch events.

API reference documentation


Utilities related to your extension. Get URLs to resources packages with your extension, get the Window object for your extension's pages, get the values for various settings. Note that the messaging APIs in this module are deprecated in favor of the equivalent APIs in the runtime module.

API reference documentation


Some common types used in other WebExtension APIs.

API reference documentation


Finds text in a web page, and highlights matches.

API reference documentation


Use the history API to interact with the browser history.

API reference documentation


Functions to internationalize your extension. You can use these APIs to get localized strings from locale files packaged with your extension, find out the browser's current language, and find out the value of its Accept-Language header.

API reference documentation


Use the identity API to get an OAuth2 authorization code or access token, which an extension can then use to access user data from a service which supports OAuth2 access (such as a Google or a Facebook account).

API reference documentation


Find out when the user's system is idle, locked, or active.

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Get information about installed add-ons.

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Add items to the browser's menu system.

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Display notifications to the user, using the underlying operating system's notification mechanism. Because this API uses the operating system's notification mechanism, the details of how notifications appear and behave may differ according to the operating system and the user's settings.

API reference documentation


Enables extensions to implement customised behavior when the user types into the browser's address bar.

API reference documentation


A page action is a clickable icon inside the browser's address bar.

API reference documentation


Extensions need permissions to access more powerful WebExtension APIs. They can ask for permissions at install time, by including the permissions they need in the permissions manifest.json key. The main advantages of asking for permissions at install time are:

API reference documentation


The pkcs11 API enables an extension to enumerate PKCS #11 security modules, and to make them accessible to the browser as sources of keys and certificates.

API reference documentation


Access and modify various privacy-related browser settings.

API reference documentation


Use the proxy API to register an extended Proxy Auto-Configuration (PAC) file, which implements a policy for proxying web requests. This implementation deviates from standard PAC design in several ways because the de-facto specification for PAC files hasn't changed since its initial implementation circa 1995. There is no standards body maintaining the specification.

API reference documentation


This module provides information about your extension and the environment it's running in.

API reference documentation


Use the sessions API to list, and restore, tabs and windows that have been closed while the browser has been running.

API reference documentation


Gets and sets properties of an extension's sidebar.

API reference documentation


Enables extensions to store and retrieve data, and listen for changes to stored items.

API reference documentation


Interact with the browser's tab system.

API reference documentation


Enables browser extensions to update the browser theme.

API reference documentation


Use the topSites API to get an array containing all the sites listed in the browser's "New Tab" page.

API reference documentation


Defines the BrowserSetting type, which is used to represent a browser setting.

API reference documentation


Add event listeners for the various stages of a navigation. A navigation consists of a frame in the browser transitioning from one URL to another, usually (but not always) in response to a user action like clicking a link or entering a URL in the location bar.

API reference documentation


Add event listeners for the various stages of making an HTTP request. The event listener receives detailed information about the request, and can modify or cancel the request.

API reference documentation


Interact with browser windows. You can use this API to get information about open windows and to open, modify, and close windows. You can also listen for window open, close, and activate events.

API reference documentation

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