Mozilla Roadmap

The development team of the Mozilla browser (Netscape Communicator 5.0)
chose the library
GTK+ as the primary one for development under XWindow,
thus replacing the commercial Motif.

The GTK+ library was created during
the development of a graphic editor GIMP.
and is now used in the project GNOME
(to create a free graphical environment for UNIX).

Details at mozilla.org, MozillaZine.

Source: linux.org.ru