Exercising Software Freedom on Firefox

I don't understand why people dislike XUL so much. In the "HTML was not intended for application GUIs but for documents" debate (which is at least currently popular on hacker news), XUL would be the answer of the FOSS XML ecosystem, a hybrid between classical gui layout engines/gui toolkits (Gtk/Qt/MFC/Cocoa/whatever) and all the XML/JavaScript ecosystem web Devs like.

Maybe XUL is regarded outdated like Java Swing, but that's just a matter of software maintenance, isn't it? I really don't understand what's bad at XUL.