Contributor's Guide

This is an archived page. It's not actively maintained.


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 (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.



Classes and Inheritance
A class is a blueprint from which individual objects are created. These individual objects are the instances of the class. Each class defines one or more members, which are initialized to a given value when the class is instantiated. Data members are properties that allow each instance to have their own state, whereas member functions are properties that allow instances to have behavior. Inheritance allows classes to inherit state and behavior from an existing classes, known as the base class. Unlike languages like C++ and Java, JavaScript does not have native support for classical inheritance. Instead, it uses something called prototypal inheritance. As it turns out, it is possible to emulate classical inheritance using prototypal inheritance, but not without writing a significant amount of boilerplate code.
Content Processes
A content process was supposed to run all the code associated with a single tab. Conversely, an add-on process was supposed to run all the code associated with a single add-on. Neither content or add-on proceses were ever actually implemented, but by the time they were cancelled, the SDK was already designed with them in mind. To understand this article, it's probably best to read it as if content and add-on processes actually exist.
Getting Started
Learn how to contribute to the Add-on SDK.
A module is a self-contained unit of code, which is usually stored in a file, and has a well defined interface. The use of modules greatly improves the maintainability of code, by splitting it up into independent components, and enforcing logical boundaries between them. Unfortunately, JavaScript does not yet have native support for modules: it has to rely on the host application to provide it with functionality such as loading subscripts, and exporting/ importing names. We will show how to do each of these things using the built-in Components object provided by Xulrunner application such as Firefox and Thunderbird.
Private Properties
People have come up with several ways to emulate private properties using existing language features. This article discusses two common techniques: one using prefixes, the other closures.