That is why they say "fire blocking". It just blocks the rapid spread of fire products (smoke) and delays the actual spread of the fire so you have time to get out. If the wall cavity is really fairly well sealed the fire might choke itself out before it ever really gets loose. It is not really that easy to light the side of a 2x4. If you have foam in there the foam itself may be the biggest accelerant for the fire. It certainly will not help the smoke problem.
