Considering that OS4 isn't going to be ported to m68k at all, and [I think, not sure, could be wrong] MorphOS only runs on PPC as well, what's the point in writing Mozilla for m68k?
There is a technical reason for it not to work on a 68030. Infact, quite a few. First off, Firebird really is very slow on a 166MHz x86 box, let alone Mozilla. Secondly, it costs significantly in developer time to port to another architecture.