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XULRunner is a runtime environment developed by the Mozilla Foundation to provide a common back-end for XUL 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, and alpha versions based on Mozilla 1.9 were released in 2007. The most recent releases of XULRunner are in the 1.9.1 branch, using the most recent Gecko engine, and are built from the same source code snapshot as Firefox 3.5.
All XUL-based applications like Mozilla Firefox, Mozilla Thunderbird, Songbird, Flickr Uploader, SeaMonkey, Conkeror, Sunbird, Miro, Joost, and TomTom Home 2.0 run on XULRunner. Beginning with version 3.0, Mozilla Firefox is actually built on top of a "private" XULRunner.
The fourth installment of the computer game series Simon the Sorcerer, Simon the Sorcerer 4: Chaos Happens, uses XULrunner.
The eMusic web site has a download application called eMusic Remote that uses XULRunner.
Google Adwords editor uses XULRunner, as does the Evergreen ILS, a free and open-source library automation system.
In addition, the XULRunner package provides access to ActiveX Control Functionality previously found in a (now defunct) 3rd party ActiveX Control built off of the Mozilla Source. Applications which use this API may function with XULRunner installed and registered.
IBM deployed XULRunner starting with Lotus Notes version 8.5.1 to provide Notes client support for XPages applications.