I have tried emerge mozilla and i also tried the most recent ebuild listed (1.2). They both freeze my entire machine when it gets to the decompressing phase. This is a very fresh install (just finished this morning).
Sorry for wasting your time, it was because of a faulty driver in the kernel. However emergeing the newest mozilla emerges without error, but it will not run.
Add USE flags, CFLAGS, strace, or anything that can help and reopen if you are interested in trying to resolve this. Try a vanilla kernel ?