Why? Why the XML UI? I can understand Mozilla's reasoning, to an extent. But now Microsoft is doing it with XAML (expected to be part of Longhorn), and there are things like Laszlo (which is pretty cool - I'm happy that the one demo applet I tried worked in Safari, pretty quickly on a G3/400 system) and apparently Macromedia is cooking something up too.
I think there are certainly some types of applets and applications where this can work. But as a central development scheme? I don't know.
All one has to do is compare native Mac OS X browsers like Safari and OmniWeb to Firebird and Mozilla on Mac OS X to notice the difference - Safari and OmniWeb just respond better and behave more predictably to a mac user. The half-native Camino browser sits somewhere in the middle (its widgets feel a little off, but I may be getting too used to the power of Cocoa's text system). Native is better.
Microsoft's XAML looks like a road gone down before - particularly HTAs (HTML Applications). I wonder how successful that's been - like Active Desktop.