Khuutra (assuming he's the firefighter) doesn't have to know who the arsonist is, he only has to guess who the arsonist is gonna target and protect him. We can assume that, since we don't know how Gnizmo died, either Khuutra guessed right the Arsonist target or the Doctor guessed right the Mafia target (though, going by Khuutra's reaction in the thread, i'm leaning for the former)