The Flamethrower so the anti air Tower are considered as bad towers so do I in someway including the fire effect.

Flamethrower: The cold fire ability is actually not a bad idea but it doesn't work out very well because low damage and rare usefulness. I think many prefer the freeze effect to be not deleted by fire. Many prefer it maybe to be replaced by something else. Another thing is that the arc is way too small it. You could make the arc size a special Attribute and a research thing for it too.
I thought of an ability just while I'm talking about it. I call it: Molotov. The Flamethrower creates a fire field in its range that damages enemies walking through it.

Anti Air: His Damage, lvl.10 Ability and his special fire Attribute suck pretty much because Bullet damage is a bit low (fire is ok), lvl. 10 Ability does like really low damage so it's basically useless(and I prefer it to be active even on enemies which died because of fire). Fire effect has the same freeze + fire problem as the flamethrower above.

Fire effect improvement idea: Maybe you can make an |Aura of heat/on touch effect with other enemies| that damages affected enemies while the enemy is on fire.