The firewall is the area between the engine bay and interior of the car. There are components such as the heater box, wiring, etc... that protrude through the firewall. After these items were installed, it was typical for the edges of the items to be sprayed with a black sealer. This was normally done before the engine was installed, but with pretty much everything installed going through the firewall. Of course, this area should have been black (or mostly black), before the sealer was applied. Some folks refer to it as sound deadener, but the results are the same.