Because, when all else fails, put the damn crabs on fire! That'll show 'em. Vermin native to the Elemental Plane of Fire, fire crabs come in two forms: the Small CR 1/2 lesser fire crab and Large CR 3 greater fire crab. Both exude heat from their shells and have vice-like grips that deal both constriction and fire damage. They are said to dwell in volcanic lakes and rivers of liquid fire, so...do with them what you will, I guess. Seriously, what is up with the Plane of Fire? Is its ecology just "Earth's oceans, but in lava!"?
Oh, hey, I saw the cinder ghoul too.
Also known as the burning leviathan, fire whales are the icing on the "liquid
SPACE!!fire works like liquid water" cake. They are CR 10 Magical Beasts that are basically baleen whales that happen to be able to swim in lava and spray scalding hot air out of their blowholes. Again, I'm not quite sure I understand the Plane of Fire's ecology. At all.