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.

From Firefox 53 onwards, no new legacy add-ons will be accepted on (AMO).

From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.

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 WebExtensions if they can. See the "Compatibility Milestones" document for more.

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.

Contributor's guide

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
Learn how classes and inheritance can be implemented in JavaScript, using constructors and prototypes, and about the helper functions provided by the SDK to simplify this.
Private Properties
Learn how private properties can be implemented in JavaScript using prefixes, closures, and WeakMaps, and how the SDK supports private properties by using namespaces (which are a generalization of WeakMaps).
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.

SDK infrastructure

Module structure of the SDK
The SDK, and add-ons built using it, are of composed from reusable JavaScript modules. This explains what these modules are, how to load modules, and how the SDK's module tree is structured.
SDK API lifecycle
Definition of the lifecycle for the SDK's APIs, including the stability ratings for APIs.
Program ID
The Program ID is a unique identifier for your add-on. This guide explains how it's created, what it's used for and how to define your own.
Firefox compatibility
Working out which Firefox releases a given SDK release is compatible with, and dealing with compatibility problems.

SDK idioms

Working With Events
Write event-driven code using the the SDK's event emitting framework.
Content scripts guide
An overview of content scripts, including: what they are, what they can do, how to load them, how to communicate with them.
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.

XUL migration

XUL Migration Guide
Techniques to help port a XUL add-on to the SDK.
XUL versus the SDK
A comparison of the strengths and weaknesses of the SDK, compared to traditional XUL-based add-ons.
Porting Example
A walkthrough of porting a relatively simple XUL-based add-on to the SDK.

Multiprocess Firefox

Multiprocess Firefox and the SDK
How to check whether your add-on is compatible with multiprocess Firefox, and fix it if it isn't.

Join the Add-on SDK community

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Document Tags and Contributors

 Contributors to this page: wbamberg, evold
 Last updated by: wbamberg,