Firestorm’s powers are very broad, and he can do a lot of tricks to disable or otherwise put the kibosh on Iceman. But Iceman is an Omega level mutant, and he can generate a cold field intense enough to shut down Firestorm’s internal nuclear reaction.

Firestorm can deal very effectively with cold powers (Killer Frost, natch), but Iceman’s power level is way beyond that of Killer Frost. But at the same time, Iceman is not necessarily brainy enough to deal with Firestorm’s transmutation (I am thinking of Jason creating anti-freeze to take out Killer Frost).

So really it could go either way, honestly. Both of these guys bring some real firepower to a fight.