Look at Wikipedia site about XUL
Look at Wikipedia site about JSF
JSF is now a part of JEE.
Look at Wikipedia site about AJAX
xulfaces is based on Java Server Faces,AJAX and XUL.
It gives you an opportunity to uses XUL and Java at the same time to build rich client UI with light deployment.
xulfaces supports Facelets 1.0.14.
Check "Using facelets" for details.
XUL is a Mozilla technology. It is not supported by Microsoft.
xulfaces uses JSF 1.2 PFD. It's an incomplete implementation.
Glassfish provides an implementation of JSF 1.2, xulfaces will use it as soon as possible.