After defeating the Rock Giant, Lion-O said to the others, "It's like a steam bath in here." However, he wasn't "in" anywhere. They were outside standing next to the moat.
When the Rock Giant breathed fire on the ThunderTank, nothing was damaged. The Thundercats were able to get in afterwards and drive away with no problems. When the Rock Giant breathed fire on Cats Lair though, it shorted out their controls.
Mumm-Ra: Hmm... Another temporary setback. Well, I'll just have to try again. There's more than one way to skin a cat.
(fleeing in the ThunderTank from the Rock Giant)
Lion-O: I don't like to run away from anything!
Wilykat: That's not anything, Lion-O. That's... That's awesome.
Lion-O: Hey, Snarf! Where are the Thunderkittens?
Snarf: Lion-O, I don't know! Don't I have enough trouble looking after you?
Mumm-Ra: Thunder? Lightning? Storm?! And fire? Something evil stirs deep in Third Earth.