I have been trying to figure this out for a while now. If, how critical mass works, is to multiply your chance to crit with Fireball, Pyro, FFB, and Scorch by 1.3 than I assume it would calculate it after buffs, and crit suppression. I'm not 100% sure on that, though. Real game testing (skada, recount, actual crit %) is probably needed... Unless, that information is out on the internet somewhere and I haven't found it yet.
I'm still frost at the moment, haven't had the time aside to regem/forge/enchant my stuff to fire, even though I'm at a good spot to switch. With the release of Bioshock today I won't have much time for WoW outside of raids anyway.