XUL ( ZOOL), which stands for XML User Interface Language, is a user interface markup language developed by Mozilla. XUL is an XML dialect for writing graphical user interfaces, enabling developers to write user interface elements in a manner similar to web pages.

XUL applications rely on the Mozilla codebase or a fork of it. The most prominent example is the Firefox web browser. However, Mozilla has reduced the usage of XUL in Firefox after their rewrite of the browser engine in version 57. This resulted in the removal of add-on customization. Firefox originally permitted add-ons to extensively alter its user interface, but this capability was removed in 2017 and replaced with the less-permissive WebExtensions API. Several forks of Firefox retain support for XUL and XUL-based add-ons. Waterfox maintains a fork of the Mozilla codebase for the Waterfox Classic browser. The Pale Moon browser, Basilisk browser, Hyperbola operating system, and several smaller projects collectively use and maintain the Unified XUL Platform (UXP) implementation.