Question: What is a Firewall? 

Answer: The firewall is the flat plate or wall like component that the engine mounts are attached to. All propeller driven model airplanes have a firewall. It is located near the front of the model just behind the engine.

Better Answer: The term firewall comes from the first use of internal combustion engines in cars and aircraft and dates back to even earlier usage in the 1800's. If we take the word apart into its elements it is exactly what is says it is... a Fire Wall.

Internal combustion engines like model airplane engines... burn fuel. They get hot, very hot. This heat is ok for the engine but is not generally a good thing for the rest of the aircraft. The Fire Wall separates the hot, heavy engine from the rest of the aircraft.

In most propellor driven aircraft the firewall is just aft of the engine and is a wall that pretty much caps the forward end of the fuselage. The engine mounts are attached to the forward face of the firewall and the engine in turn is attached to the engine mounts.

In VMAR models the firewall is generally a piece of plywood that looks like a small flat plate. The firewall may be round or elliptical, or be squarish in shape with rounded corners and measure 3 to 8+ inches across. It is usually about 6mm (1/4") thick.