XULRunner is a discontinued, packaged version of the Mozilla platform to enable standalone desktop application development using XUL, developed by Mozilla. It replaced the Gecko Runtime Environment, a stalled project with a similar purpose. The first stable developer preview of XULRunner was released in February 2006, based on the Mozilla 1.8 code base. Mozilla stopped supporting the development of XULrunner in July 2015.

XULRunner was a "technology experiment", not a shipped product, meaning there were no official XULRunner releases, only stable builds based on the same code as a corresponding Firefox release.

XULRunner is a runtime that can be used to bootstrap multiple XUL + XPCOM applications that are equal in capabilities to Firefox and Thunderbird.

XULRunner stores a variety of configuration data (bookmarks, cookies, contacts etc.) in internally managed SQLite databases, and even offer an add-on to manage SQLite databases.

Mozilla Firefox, Mozilla Thunderbird, Nightingale, Songbird, Flickr Uploadr, SeaMonkey, Conkeror, Sunbird, Miro, Joost, and TomTom Home 2.0 ran on XULRunner. Starting with version 3.0, Mozilla Firefox uses a "private" XULRunner, meaning the framework is installed locally in the application directory.

Kiwix, an offline browser for Wikipedia and Project Gutenberg, used XULRunner until 2017.

The Google AdWords Editor used XULRunner until 2010, as did Evergreen ILS, an open-source library automation system.

The fourth version of the video game series Simon the Sorcerer, Simon the Sorcerer 4: Chaos Happens, uses XULRunner.