Wikipedia
XULRunner
XULRunner is a run-time engine for XUL. 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. It is developed by the Mozilla Foundation to provide a common back-end for XUL-based applications.
XULRunner is a "technology experiment", not a shipped product, meaning there are no "official" XULRunner releases, only stable builds based on the same code as a corresponding Firefox release.
Mozilla stopped supporting the development of XULrunner in July 2015.