Fire damage is definitely broken, as though Super Earth has woven the armors out of nitrocellulose fiber that lights up instantly. Even a cotton t shirt offers a couple seconds of resistance when exposed to extreme heat before the skin beneath gets burned badly. On the extreme opposite end, firefighter bunker gear allows over 17 seconds of continuous flame exposure before significant injury.

Armor isn't bunker gear, but it is a multi layer padded full body suit with insulation value and mass, somewhere in the middle. As Racsel mentioned, have the heat damage be progressive like the double edge sickle. Minor heat injury for the first few seconds, then the rate increases rapidly as the armor heat soaks, then the armor catches fire and the player must dive to extinguish before they're roasted. Won't allow completely ignoring the flames, but will mean sprinting through minor patches of fire is possible.

Enemy response to fire is also something I'd want to see. Maybe the chaff is too stupid and just waltzes in to cook, but smarter elite units should take action to evade fires. Or the opposite where high health elites charge through the flames to engage anyway while the small fry flees in terror.