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.
This page lists more theoretical in-depth articles about the SDK.
- Getting Started
- Learn how to contribute to the SDK: getting the code, opening/taking a bug, filing a patch, getting reviews, and getting help.
- Learn about the module system used by the SDK (which is based on the CommonJS specification), how sandboxes and compartments can be used to improve security, and about the built-in SDK module loader, known as Cuddlefish.
- Classes and Inheritance
- Private Properties
- Content Processes
- The SDK was designed to work in an environment where the code to manipulate web content runs in a different process from the main add-on code. This article highlights the main features of that design.
- Testing the Add-on SDK
- Learn how to run the Add-on SDK test suites.
- Module structure of the SDK
- SDK API lifecycle
- Definition of the lifecycle for the SDK's APIs, including the stability ratings for APIs.
- Two Types of Scripts
- This article explains the differences between the APIs available to your main add-on code and those available to content scripts.
- Porting Example
- A walkthrough of porting a relatively simple XUL-based add-on to the SDK.
- Multiprocess Firefox and the SDK
- How to check whether your add-on is compatible with multiprocess Firefox, and fix it if it isn't.