XULRunner is not a frozen API but instead delivers a set of XUL toolkits. Some of the XULRunner applications will only work on specific versions of toolkits. The current version number convention of XULRunner is meant to match the number of the corresponding Gecko milestone.
Developers can also be allowed to specify what the minimum is and maximum version of the toolkit is required to run the specific application. This is currently implemented as an option in the .xulapp file on Macs and in the application.ini file for Windows and Linux. The applications version is specified the MinVersion and MaxVersion. XULRunner will check this constraint on the application and refuse to run those who do not pass the version test.
This version includes new security/stability updates which are available as a downloadable patch. It replaces the previous version 188.8.131.52. All XULRunner users should upgrade. This patch can be found at the Mozilla Development Center under XULRunner 184.108.40.206 Release Notes
The source code for this version can be found on the following link, xulrunner-220.127.116.11-source.tar.bz2
XULRunner 18.104.22.168 was the first stable developer preview release. It features include launching standalone XUL applications and embedding APIs. This can be used for rendering web pages within native and Java applications. This version can be downloaded at the Mozilla Development Center under XULRunner
The source code of this version can be found on the following link, xulrunner-22.214.171.124-source.tar.bz2