I have published XUL Schema on CodePlex. It is fairly complete and it should work correctly for most scenarios. Get the latest source code and follow instructions from the home page. Here are some screenshots.
All comments are welcome. If something is not working correctly please report and I'll surely look in to it.
Again, consider this work in progress, it should work correctly for most of the scenarios but there are scenarios where it will break - for example templates with HTML contents. While I'm developing my extensions I keep updating the schema so from time to time check out the latest version of the code.