When I run mozilla in GNOME, it doesn't have an icon. Other applications have one, e.g. XChat, Sylpheed. Don't know if this is a GNOME, Mozilla or Gentoo-Bug. (I use mozilla-1.1-r1)
Maybe this helps to resolve the bug: Today i recompiled mozilla without gtk2 and now I have an icon. (Quite strange, since gnome2 is gtk2 and it only works with the non-gtk2 mozilla nice together).
Not really ... the Icon is a .png or .xpm (cant remember), and thus is gtk+ version independant.
Fine with mozilla-1.2.