I came across XULrunner (GUI rendering backend used by Firefox and Thunderbird), and thought the concept is really interesting - building GUIs in XML. I read the Wikipedia page, and thought it was as clear as mud. I read the intro documentation and tutorial too on the Mozilla Developer Centre.

They all emphasize that XUL is only for the GUI, and has nothing to do with the application logic. The tutorial on MDC only covered how to create a simple UI, but my big question is, how do I interface the UI with code? In what language? The tutorial seems to suggest Javascript... but I find it hard to believe that Firefox is written in Javascript entirely.

Anyone has experience using XULrunner?