The fire zombies, whatever they are, aren't so much the problem as your dumb friends. They casually wander into the fire, in doing so go berserk, then hug the edges of the screen making it difficult to see where they are to put them out in time. The zombies only come into play when your friends wander into a pack of them and then burst into flames and you have no way to save them. You quickly reach a critical mass of zombies if more than two of your friends (or strangers? who are these people, really, and why am I tasked with constantly protecting these idiots as though my character is the equivalent of Holden Caufield with a throw quilt?) succumb to the flames. The fact that saved people return so quickly actually is the biggest problem... you don't have time to thin the herd, really, before they all gleefully amble right back into the fire.
Is there some message about futility in here? If so, bravo. If not, you can't take credit for it now but it made me feel that way. :-p