There are a lot of fire-based characters, so I know that I am missing very many. If you point out some, I'll add them. They are all in character. Combatants may only use their fire attacks to harm one another, but they can use other abilities to aid themselves.
Combatants: Fire, Magma, Firefly, Ozai (No Sozin's Comet), Cinder (Killer Instinct), Kimiko (Xiaolin Showdown), Pyro, Hot-Streak, Portgas D. Ace.
Round 1: Fight takes place in a concrete arena.
Round 2: Fight takes place in a cryogenic factory.
Round 3: Fight takes place in an oil refinery.
Winner is the last one alive.
Human Torch was removed because he is too powerful.