TBO, I think the Genius just didn't want the hassle of trying to find the problem and fix it (i.e. "reset" Safari, delete prefs, remove caches etc..) and assumed Firefox was "clean" on your System... So it simply was a quick workaround IMHO.
Smart.
I have never encountered problems on any Apple-site (developer included) with Safari.
well if you don't renew your Developer account then they're losing a profit? so they'd rather you use Firefox to do it than not renew it at all.. correct me if i'm wrong?