[thejeff]

You don't have too many fire gems until research is done. Until then they go into Lanterns.

The problem with Fire is that it has little other than fire damage, so it's almost completely useless against Fire resistant things. And all thugs, SCs and mages coming against you are likely to be fire resistant.
It's powerful, but it's a one-trick pony. The AI doesn't know to go for fire resistance, so it's great in SP.

Other paths have useful buffs or other spells that work even if the opposition is immune to the path's main damage. Even against shock resistant enemies, Air thugs can use mistform & Mirror Image, armies can use Fog Warriors & Mass Flight. Thugs/SCs can Cloud Trapeze.
What do you do with your fire magic against 100% FR?