Reaction score: 189
There's a lot of software that uses gtk and qt on top of windows (even some games do that, to be even worse, some games uses qtwebkit/webkit-gtk).I don't really know what UI toolkit Firefox uses on Windows. I would be surprised if it was Gtk or Qt. They probably have a light abstraction layer ontop of Win32.
wxWidgets I suppose has a similar approach. Unfortunately on Linux/BSD, it sticks ontop of Gtk or Qt so you lose that "lightness".