any thoughts about mozilla engine / XUL?

I want to write a small application to upload multiple pictures to those free image hosting providers out there like flickr, imageshack, photobucket, etc.

I thought about using xul to give it a try. The alternatives are wxwidgets or GTK. But i would really like to give a try to XUL.

Is anybody out there using it?
I haven't really understand how the application is packaged 100%. Does anybody knows if there is any gateway that allows me to use any language? i heard i could only use java or C/C++ . I still yet to understand where that hypothetical restriction would come from.

Any info or advice is appreciate.