A "firewall" seperates the passenger compartment from the engine.

In the event of a fire in the engine, it helps keep the passengers safe.

It "walls off" the fire....

So cars, planes, trucks, etc. all have firewalls.

Since the term is so common, it has stuck with RC aircraft as well.

We name the airplane parts exactly the same as full size aircraft... e.g. elevators, ailerons, firewall, cowl, canopy, etc.