The Wikipedia page in XULrunner is quite informative for a starting point.
And from
https://wiki.mozilla.org/XUL:Xul_RunnerVersioning
XULRunner is a delivery vehicle for the XUL toolkit, which is not a frozen API. It is an API that has historically evolved over time, and it will likely continue to evolve for some time to come. While people agree that we need to stablize that API, it will not happen overnight.
For these reasons, it is important that XULRunner support applications that require specific versions of the toolkit. The current thinking is that XULRunner will be versioned (with version number matching the corresponding Gecko milestone), and applications will be able to specify the version(s) of the toolkit that they require.
This is in fact already implemented as options in the .xulapp file. Applications can specify a MinVersion and MaxVersion for the toolkit versions they require. XULRunner will refuse to load an application that does not pass the version test.