Q: Does the Forest Fire Creeper “burn” one Keeper and then get discarded, or does it slowly burn all of that player’s Keepers?

A: The Forest Fire Creeper says “If you have this at the *start* of your turn, discard a Keeper you have on the table. Discard this if you have no Keepers in play.”

Looks like nothing says you get to discard it after it burns only one of your Keepers. You only get to discard it “for free” if you have no Keepers in play, in other words, until it has “burned itself out” by using up all the fuel. You might conceivably “put it out” by using some other card to get rid of it, but if you haven’t got any way to do that, then it will just burn through your Keepers until they are all gone. But then it will go away, and you can safely play Keepers again.