Add-ons aren't just for Firefox. Learn how to extend other Mozilla applications, such as the Thunderbird mail client and Firefox for mobile devices.
Developing Add-ons for Thunderbird
This tutorial will take you through the steps required to build a very basic extension - one which adds a status bar panel to the Thunderbird Mail Client containing the text "Hello, World!"
This article describes the Thunderbird user interface, discusses example modifications, and introduces some of the APIs most commonly used by extension developers.
This section contains many code samples and how-to articles on common Thunderbird extension tasks.
If you can't find what you're looking for in the above articles, try the Mozilla Developer Center's Thunderbird extensions landing page.
Developing Add-ons for Mobile
This document describes the architecture of Firefox on mobile and includes performance tips for mobile code.
This document explains the specifics on creating an extension for mobile.
Mobile extensions have small screen space and limited resources to work with. This document explains best practices for designing and developing for a mobile environment.
Code snippets specific to mobile.
How to design an interface for your mobile extension.