TB has four passive abilities. Two of those don't level up. You gain the effect of one when you change from fire to ice, and the other from ice to fire. Both last 20 seconds, so if you change form every 19 seconds, you can keep them up indefinitely. They're +50% mana recharge and +50 weapon damage.
The other two are ONLY active when you are in the right mode. So no, no permafrost in fire mode.