Archived add-ons documentation.

Add-on SDK
Using the Add-on SDK, you can create Firefox add-ons. You can use various standard Web technologies: JavaScript, HTML, and CSS, to create the add-ons. The SDK includes JavaScript APIs, which you can use to create add-ons and tools for creating, running, testing, and packaging add-ons.
Exports bookmarks to an HTML bookmark file.
Imports bookmarks from an HTML bookmark file.
Bootstrapped extensions
browser.bookmarks.export( function() {...} // optional function )
Code snippets
Creating custom Firefox extensions with the Mozilla build system
There is a wealth of material on creating extensions for Firefox. All of these documents currently assume, however, that you are developing your extension using XUL and JavaScript only. For complex extensions, it may be necessary to create components in C++ that provide additional functionality.
Extension Etiquette
This article describes best practices when making extensions, including how to be kind to your users. It assumes that you are already familiar with Building an Extension.
Extension Packaging
Extensions are packaged in a form of Installable Bundle which can be downloaded and installed by a user, or provided pre-packaged with an application or by an external program. Extensions use a directory structure which can provide chrome, components, and other files to extend the functionality of an XUL program.
Extensions support in SeaMonkey 2
Starting with SeaMonkey 2 Alpha 1 SeaMonkey supports toolkit/-style extensions. These type of extensions have many advantages for both users and developers compared to the old xpinstall/-style extensions.
Firefox addons developer guide
The original document is in Japanese and distributed via the website. So there may be still some reference to the xuldev website (we want to host source code on MDC, not on xuldev), and to Japanese things (like some specific locales, which have been translated to French since non-latin characters are not well supported).
Hotfix Extension
This document has been moved to the Add-ons wiki.
How to convert an overlay extension to restartless
First off, what kind of add-on are we talking about here? Well, XUL overlays and windows, JSM files, chrome & resource mappings with localization, default preferences, but no XPCOM components of your own. Some of that will have to be replaced and the rest will need to be loaded differently.
Inline options
Firefox 7 supports a new syntax for defining extensions' preferences for both bootstrapped and traditional extensions. The user interface for the preferences defined with this new syntax appears in the extension's detail view in the Add-on Manager. This functionality originally appeared in Firefox mobile and is now available in Firefox on the desktop as well.
Install Manifests
An Install Manifest is the file an Add-on Manager-enabled XUL application (e.g. Firefox or Thunderbird) uses to determine information about an add-on as it is being installed. It contains metadata identifying the add-on, providing information about who created it, where more information can be found about it, which versions of what applications it is compatible with, how it should be updated, and so on.
Interaction between privileged and non-privileged pages
An easy way to send data from a web page to an extension is by using custom DOM events. In your extension's browser.xul overlay, write code which listens for a custom DOM event. Here we call the event MyExtensionEvent.
Legacy Add-ons
This section contains links to documentation for legacy technology for add-on development, including:
Legacy extensions for Firefox for Android
Add-ons that work with desktop Firefox do not automatically work in Firefox for Android:
Overlay extensions
This page contains links to documentation for the approach to developing extensions for Gecko-based applications which uses:
Performance best practices in extensions
One of Firefox's great advantages is its extreme extensibility. Extensions can do almost anything. There is a down side to this: poorly written extensions can have a severe impact on the browsing experience, including on the overall performance of Firefox itself. This article offers some best practices and suggestions that can not only improve the performance and speed of your extension, but also of Firefox itself.
Security best practices in extensions
This document is intended as a guide for developers to promote best practices in securing your extension. Your goal is to keep your users safe. Some items mentioned are strict guidelines, meaning that if you don't follow them then your add-on will not be approved on Mozilla add-ons. Other items are recommendations. The difference will be clearly flagged.
Setting up an extension development environment
This article provides suggestions for how to set up your Mozilla application for extension development. These details apply to Firefox, Thunderbird, and SeaMonkey (version 2.0 and above).
Tabbed browser
Here you should find a set of useful code snippets to help you work with Firefox's tabbed browser. The comments normally mark where you should be inserting your own code.