When you actually get rank V fire magic the tooltip shows that heart of fire is one of the unlocked spells, however the spell is available to the player with only one rank in fire magic.
Edit: Ignore what I posted there.. I must have been on acid or something. In any case, this is possibly an issue with immediate unit strength. Additional damage to troops scales exceptionally well, as should be apparent by the incredibly small increments in weapon damage. This wouldn't be so bad (it jumps you 1-2 tech levels) but fire damage is not mitigated by defense. Currently I'm pumping troops out of enclaves because they are just plain better than fortress units due to this spell.
Is +1 fire damage per essence too much or is it just that it is available so early?