The XML User Interface Language (XUL) is a markup language for describing user interfaces. With XUL you can create rich, sophisticated cross-platform web applications easily.

Extensions are small add-ons that add new functionality to Mozilla applications such as Firefox and Thunderbird. They can add anything from a toolbar button to a completely new feature. They allow the application to be customized to fit the personal needs of each user if they need additional features, while keeping the applications small to download.