To do the job properly the firewall should be sealed at all possible apertures including at the mating of the firewall to the fuselage skins. This serves 3 purposes. 1. It prevents flames and heat from entering the cabin (for a while at least) in the event of a fwf fire. 2. It prevents smoke from entering the cabin in the event of FWF fire. 3. It prevents carbon monoxide from entering the cabin in the event of an exhaust system leak.
It might be quite difficult to do this if the skins have already been rivetted on.
Incidentally proseal is NOT the sealant to use on the firewall. Despite the fact that it's used on fuel tanks it is quite combustible...and when it burns it gives off copious amounts of black smoke.
The trouble with many RV builders is that they start off using proseal to do their tanks and then proceed to use the stuff for every other possible sealant application.
In fact Vans sells the an appropriate sealant. It's called FlameMaster CS1900 Firewall Sealant. It can cope with a flash temperature of 2000 F. See
http://www.vansaircraft.com/cgi-bin/catalog.cgi?ident=1193705588-328-495&browse=misc&product=cs1900