Didn't work in Firefox on a Mac in OS X, either. BUT, to get Firefox to accept PNG's at all in OS X meant forcing a file association on it. One of the members of a Yahoo group I belong to uses png's all the time. The first time I downloaded one Panther acted like the file was poison or something. After control-click/get info and forcing it to open with FF, all was well. But his png's are not animated.
I wonder if animated png's are benign compared to Flash gifs. I block all Flash by default, in part because it's so cpu intensive.