XUL (XML User Interface Language) is Mozilla's XML-based language that lets you build feature-rich cross platform applications that can run connected or disconnected from the Internet. These applications are easily customized with alternative text, graphics and layout so they can be readily branded or localized for various markets. Web developers already familiar with Dynamic HTML (DHTML) will learn XUL quickly and can start building applications right away. Open XUL Periodic Table in Firefox or another Gecko-based browser to see some XUL demos.
Note: This "XUL Periodic Table" cannot be shown in recent versions of Gecko because it uses remote XUL, a technology whose support was dropped because of potential security issues.
A comprehensive tutorial on Mozilla add-on development.
A list of changes in Firefox 3.0, affecting XUL developers.
A list of changes in Firefox 2.0, affecting XUL developers.
A list of changes in Firefox 1.5, affecting XUL developers.