Warning: The content of this article may be out of date. This tutorial describes how to create an extension for the old versions of Mozilla Suite (currently SeaMonkey). See Building an Extension for an up-to-date tutorial (it focuses on Firefox, but is also applicable to other new applications).
A Mozilla extension is an installable enhancement to the Mozilla browser that provides additional functionality (for example Linky, which adds an item to the context menu for opening multiple links in a document or selection). This tutorial walks you through the process of building a Mozilla extension that adds an icon to Mozilla's status bar showing the current status of the Mozilla source code (i.e. whether or not the latest version of the code is compiling successfully and passing tests). The extension will access Tinderbox, mozilla.org's webtool for tracking source code status, to get the status of the code.
Completing this tutorial will give you a basic understanding of how Mozilla's user interface (UI) is constructed, how to find the source code for the UI you want to extend, how to make an installation of Mozilla modifiable, how to use Mozilla's network library to load and parse web pages in JavaScript, and how to use dynamic overlays to package a Mozilla extension for installation by others.