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.
XULRunner is a Mozilla runtime package that can be used to bootstrap rich applications such as Firefox and Thunderbird. It provides an innovative technology that developers can use to enrich and integrate their desktop applications. Applications can be written in any Mozilla supported technologies, such as HTML, XHTML, SVG, XUL, CSS, JavaScript and of course Python.