No, I am NOT counting the other Sprites in this, so don't just say "It's worse than Hexing Haze." (Though that might be true.)
Right now I have a maxed Firebolt, Flame Barrier with maxed Heavy Smoke (and 3 skill points in other areas), and an unleveled Firestorm. I have 2 unused Skill Points right now.
Right now, when criting, Firebolt does 150 damage and Strong Fire. (Fire damage is same as that of FoV.) Firestorm seems to do much less powerful Fire AND it has a poor chance of dealing Fire. Right now Firestorm is completely pointless for me.
Does it get much better when it's maxed? Especially with maxed Extra Crispy?
I have a Battle Sprite details topic for people just like yourself.
Maxing Extra Crispy puts the fire up at 1.5 strength vs Firebolt's 3 strength (and Fang of Vog's 4 strength.)
To understand exactly what that means, compare my status chart to enemy resistances.
For instance, using 3 strength fire against an enemy with -5 fire resistance results in 3 - (-5) = 8 strength fire, resulting in 5 ticks of 100% damage.