This is one of those terms that when used, car guys & gals think of the wall between the engine compartment and the vehicle’s interior.  Computer minded folks tend to think of the small computer between the outside world and their computer system designed to protect it from intruders.  Firewalls in the building industry is designed to protect life and property by slowing the spread of fire to other parts of the building or neighboring buildings.  This firewall is commonly comprised of at least 5/8” gypsum sheeting which creates a barrier that takes roughly an hour or more to burn through.  All openings in a designed firewall must also meet the “one-hour” rating, service doors are an example.