Not that many actually until Kunark. The best wizard nuke is cold, shaman nukes are cold and poison, and the high-level druid nukes are split evenly between cold and fire. The only class that really has fire as their primary damage source is magician. Then wizards get Sunstrike in Kunark, of course, but people mostly use draughts and lures by then and the best draught is also cold. The 59 druid nuke is fire, though.