I tried to figure out how to play my fire mage last night, and failed fairly miserably. I've been searching for some kind of guide, but the usual suspects have done very little or no theorycrafting. I guess the icy-veins guide provides a thorough write up, but some of the advice seems a little wrong. Idk.
I did find, however, what appears to be a nice little guide to fire maging in 5.0.4 right here.
One main tip, not present in the guide: hit cap is now 15% for all spell casters (unless you are a dranei). That comes out to 1537 hit at level 85. So you will need to ditch some hit.