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 thinking of XULRUnner version number is for it to be matching 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 can specify the MinVersion and MaxVersion for the toolkit versions that are required. XULRunner will check this constraint on the application and refuse to run those who do not pass the version test.
Is a security/stability update which is available as a patch to be downloaded. It replaces version 126.96.36.199. All XULRunner users should upgrade. This patch can be found at the Mozilla Development Center under XULRunner 188.8.131.52 Release Notes
For the source code of this version, click on the following link, xulrunner-184.108.40.206-source.tar.bz2
XULRunner 220.127.116.11 is the first stable developer preview release of XULRunner. 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
For the source code of this version, click on the following link, xulrunner-18.104.22.168-source.tar.bz2