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 addons.mozilla.org (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.
Modules listed on this page implement high-level APIs for building add-ons: creating user interfaces, interacting with the web, and interacting with the browser.
Unless the documentation explicitly says otherwise, all these modules are "Stable": we'll avoid making incompatible changes to them.
- Create a page that does not contain navigational elements.
- Data encoding and decoding using Base64 algorithms.
- Interact with the system clipboard, setting and getting its contents.
- Adds items, submenus, and menu separators to the page's context menu.
- Assign hotkey combinations to functions in your add-on.
- Exposes the IndexedDB API to add-ons.
- Display transient, toaster-style desktop messages to the user.
- Run scripts in the context of web pages whose URL matches a given pattern.
- Create a permanent, invisible page and access its DOM.
- Creates transient dialogs to implement part of an add-on's user interface.
- Interact with Firefox's Password Manager to add, retrieve and remove stored credentials.
- Check whether a given object is private, so an add-on can respect private browsing
- Utility functions for working with query strings.
- Make simple network requests.
- Get and set text and HTML selections in the current web page.
- Access data that is bundled with the add-on, and add-on metadata.
- Store preferences across application restarts.
- Lets an add-on store data so that it's retained across Firefox restarts.
- Query the add-on's environment and access arguments passed to it.
- Open, manipulate, and access tabs, and receive tab events.
- Set one-off and periodic timers.
- Provides access to button, toolbar, and sidebar UI objects.
- Construct, validate, and parse URLs.
- Provides functions for communicating with an embedded WebExtension.
- Create a simple user interface for an add-on in Firefox's add-on bar.
- Enumerate and examine open browser windows, open new windows, and listen for window events.