WebExtension JavaScript API 可以在附加组件的后台脚本和附加组件定义的任何浏览器动作或页面动作中使用。这里的部分API也可以通过附加组件的内容脚本访问（见内容脚本指南列表）。
要使用更强大的 API，您需要在您的 manifest.json 中申请权限。
您可以使用 browser 命名空间访问这些 API。
function logTabs(tabs) {
console.log(tabs);
}
browser.tabs.query({currentWindow: true}, logTabs);许多 API 为异步，返回一个 Promise：
function logCookie(c) {
console.log(c);
}
function logError(e) {
console.error(e);
}
var setCookie = browser.cookies.set(
{url: "https://developer.mozilla.org/"}
);
setCookie.then(logCookie, logError);请注意，这不同于 Google Chrome 的扩展系统，它使用 chrome 而非 browser 名字空间，并且对异步函数使用回调而不是 promises。为辅助移植，Firefox 实现的 WebExtensions 支持 chrome 和回调以及 browser 和 promises。Mozilla 也写了一个 polyfill 使使用 browser 和 promises 的代码能不经修改的在 Chrome 中使用：https://github.com/mozilla/webextension-polyfill。
微软 Edge 使用 browser 名字空间，但尚不支持基于 promise 的异步API。目前在 Edge 中，异步 API 必须使用回调。
并非所有浏览器都支持这里的所有 API：详情见浏览器对 JavaScript API 的支持。
alarms
Schedule code to run at a specific time in the future.
bookmarks
The WebExtensions 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.
browserAction
Adds a button to the browser's toolbar.
browserSettings
browsingData
Enables extensions to clear the data that is accumulated while the user is browsing.
clipboard
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.
commands
Listen for the user executing commands that you have registered using the commands manifest.json key.
contentScripts
With the contentScripts API, an extension can register and unregister scripts at runtime.
contextualIdentities
Work with contextual identities: list, create, remove, and update contextual identities.
cookies
Enables extensions to get and set cookies, and be notified when they change.
devtools.inspectedWindow
The devtools.inspectedWindow API lets a devtools extension interact with the window that the developer tools are attached to.
devtools.network
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).
devtools.panels
The devtools.panels API lets a devtools extension define its user interface inside the devtools window.
dns
downloads
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.
events
Common types used by APIs that dispatch events.
extension
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.
extensionTypes
Some common types used in other WebExtension APIs.
find
Finds text in a web page, and highlights matches.
history
Use the history API to interact with the browser history.
i18n
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.
identity
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).
idle
Find out when the user's system is idle, locked, or active.
management
Get information about installed add-ons.
menus
Add items to the browser's menu system.
notifications
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.
omnibox
Enables extensions to implement customised behavior when the user types into the browser's address bar.
pageAction
A page action is a clickable icon inside the browser's address bar.
permissions
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:
pkcs11
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.
privacy
Access and modify various privacy-related browser settings.
proxy
Use the proxy API to proxy web requests. There are two different ways you can do this:
runtime
This module provides information about your extension and the environment it's running in.
search
Retrieves search engines and executes a search with a specific search engine.
sessions
Use the sessions API to list, and restore, tabs and windows that have been closed while the browser has been running.
sidebarAction
Gets and sets properties of an extension's sidebar.
storage
Enables extensions to store and retrieve data, and listen for changes to stored items.
tabs
Interact with the browser's tab system.
theme
Enables browser extensions to update the browser theme.
topSites
Use the topSites API to get an array containing pages that the user has visited often and frequently.
types
Defines the BrowserSetting type, which is used to represent a browser setting.
webNavigation
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.
webRequest
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.
windows
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.