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.
- Getting started
- How to install the SDK and use the jpm tool to develop, test, and package add-ons.
- Interact with the browser
- Open web pages, listen for pages loading and list open pages.
- Development techniques
- Learn about common development techniques, such as unit testing, logging, creating reusable modules, localization, and mobile development.
- Contributor's guide
- Learn how to start contributing to the SDK and about the most important idioms used in the SDK code such as modules, classes and inheritance, private properties, and content processes.
- SDK infrastructure
- Aspects of the SDK's underlying technology: modules, the Program ID and the rules defining Firefox compatibility.
- Content scripts
- A detailed guide to working with content scripts.
- SDK idioms
- The SDK's event framework and the distinction between add-on scripts and content scripts.
- XUL migration
- A guide to porting XUL add-ons to the SDK. This guide includes a comparison of the two toolsets and a working example of porting a XUL add-on.
- Multiprocess Firefox and the SDK
- How to check whether your add-on is compatible with multiprocess Firefox or not and fix it accordingly.
- Low-Level APIs
- Reference documentation for the low-level SDK APIs.