XULRunner is a runtime environment developed by the Mozilla Foundation to provide a common back-end for XUL-based applications. 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.
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.
In addition, the XULRunner package provides access to ActiveX Control functionality previously found in a (now defunct) third-party ActiveX Control built off the Mozilla source code. Applications using this application programming interface (API) may function with XULRunner installed and registered.
Starting with Lotus Notes version 8.5.1, IBM deployed XULRunner to provide Notes client support for XPages applications.