In computer programming, XUL is an XML user interface markup language (developed by the Mozilla project) which operates in Mozillacross-platform applications such as Firefox and Flock. The Mozilla Gecko layout engine provides an implementation of XUL used in the Firefox browser.
Specifically XUL has no formal specification and does not inter-operate with non-Gecko implementations. However, it uses an open source implementation of Gecko, tri-licensed under the GPL, LGPL, and MPL. Mozilla provides experimental XULRunner builds to let developers build their applications on top of the Mozilla application framework and XUL in particular. XUL provides a portable definition for common widgets, allowing them to move easily to any platform on which Mozilla applications run.