XUL was used by various Mozilla projects, forks of Mozilla projects, and projects closely related to Mozilla. It was designed as an expansion of HTML to be used with applications rather than documents. When XUL was introduced, it added features beyond the then standard HTML 4. XUL is similar to web technologies implemented by the Gecko rendering engine. XUL was devised at Netscape in 1997 as part of the development effort that eventually became the Mozilla codebase. The Periodic Table of XUL Elements is a reference extension designed to demonstrate XUL's capabilities. Several forks of Firefox, such as Pale Moon, Basilisk, and Waterfox, retain support for XUL add-ons. Firefox originally permitted add-ons to extensively alter its user interface via custom XUL code, but this capability was removed in 2017 and replaced with the less-permissive WebExtensions API. The most notable example is the removal of add-on customization. However, in recent years, Mozilla has been reducing the usage of XUL in Firefox.

XUL applications rely on the Mozilla codebase (or a fork of it) the most prominent example is the Firefox web browser. 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 ( / ˈ z uː l/ ZOOL), which stands for XML User Interface Language, is a user interface markup language developed by Mozilla.

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