I remember trying to write a cross-platform interface for some software in XUL, around 2003. I even bought two (physical!) books on the subject. It was totally impossible! The libraries were ever-changing and you needed the entire Mozilla CVS tree checked out to compile anything and just getting that to compile was endless complexity. I gave up and used an early port of Gtk to Windows.

Such a "could have been great" technology if it wasn't for Mozilla ...