This tutorial is meant to be the stepping stone that will turn you into a professional Firefox extension developer in no time. We have poured years of XUL experience into it, providing many solutions for problems extension developers commonly run into.
XUL School was created by Appcoast (formerly Glaxstar), one of the few companies dedicated to building high-quality Firefox extensions. A team of over a dozen XUL developers conformed Glaxstar at the time this tutorial was created, and the combined experiences of years creating Firefox extensions are reflected here.
With this tutorial you'll learn how to develop Firefox extensions. You'll learn how to quickly do the most common tasks in extension development, comparing several different approaches to solve them. In most cases we'll provide code samples that you can easily copy and adapt to your needs, as well as some working example extensions. The tutorial aims to be as brief as possible, often falling back on Mozilla documentation for more detailed information. You can think of it as a quick guide to the expansive world that is the Mozilla platform. Most links in this documentation are meant to be clicked and read.
We'll start with a brief introduction to some key concepts, in case you're not familiar with Mozilla and Firefox. XUL (pronounced "zool") is one of many technologies used for creating Mozilla-based products and extensions. It is only one part of the development landscape, but given that it's practically exclusive to Mozilla, it tends to be used to identify all Mozilla-related development. You'll sometimes read terms like "XUL applications" and "XUL extensions", but rarely will they refer to projects that are exclusively built with XUL. It usually means that the projects were built using Mozilla technologies. Even this project, called XUL School, covers several other technologies such as JavaScript, CSS, XBL and XPCOM.XULRunner
XULRunner includes the Gecko rendering engine, the Necko networking library, and several other components that provide OS-independent file management, accessibility, and localization, among others. It is this very powerful platform that has allowed such a fast growth of the development community surrounding Mozilla and Firefox.
XULRunner is available in binary form at the XULRunner page, and it is the base for several projects, such as Songbird, Miro and Eudora. There's a very comprehensive list of XULRunner applications in the XULRunner Hall of Fame.
Gecko
The Gecko engine is the part of Firefox used to render web pages and its own user interface. You can identify the level of compatibility of web standards in Gecko-based browsers looking at their User Agent string, which should include the Gecko version. Gecko versions are somewhat independent from Firefox versions, and you can see a mapping of Firefox versions and Gecko versions at the Gecko page. The User Agent String for Firefox at the time of this writing (in US English, Mac OS X) is:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:25.0.1) Gecko/20100101 Firefox/25.0.1
The highlighted section is the Gecko version: 25.0.1. You can read and copy the user agent string of any Firefox window, choosing "Help > Troubleshooting Information" from the main menu.