HOW TO: Write Your First Firefox Extension
when you use Firefox’s coolest features, like All-In-One Sidebar or MeasureIt, you’re actually 
looking at glossy, beautiful extensions. Firefox’s extensions add specific functions, things the creators never intended, neatly integrated into the browser. The range of possibilities is boundless — making an extension is more or less like building a new application to do whatever you want, within the confines of Firefox (or Mozilla. In this article whenever I say "Firefox," you can echo "or Mozilla" to yourself in your head if you’re a Mozilla fan.)
Today let’s take a look at how one goes about creating these magical extensions. The active ingredient is XUL, a markup language (the eXtensible [or "XML-Based"] User-interface Language, to be precise) that describes things like toolbars, menus, keyboard shortcuts. It’s what Firefox uses internally, in fact, to handle its own UI (what the developer types call "chrome"). XUL will provide the interface. JavaScript handles the functionality. Your First Firefox Extension