As insanely powerful as Firestorm can be sometimes, it's well established in DC and Marvel that molecular manipulation powers work for jack on living tissue or magic, let alone both. I know Firestorm can supposedly affect living material but psychologically blocks it, but as far as feats go all he is likely to be able to do to Thor is shoot him with energy blasts. Since Thor is basically immune to electricity, heat, cold, poison and suffocation he wins this due to his being able to tear Firestorm to pieces with his bare hands, let alone Mjolnir.