Tech Talk Day!
Well son, a firewall is a wall between the engine and passenger compartments. With a front-engined vehicle, the firewall is part of the induction cowl, and ahead of the passenger compartment. With mid or rear- engined cars, it is behind the passenger compartment.
The firewall protects the passenger cabin from an engine fire. Most firewalls have carbon-fiber in them to protect from the engine flying backward into the passenger compartment.
Way back when, firewalls used to be about an inch thick, and made out of steel. The only thing that could break them was a bomb or a massive engine fire. Nowadays, they are about half an inch thick, and extremely strong.