RE: Viper firewall broke loose.
Is the firewall glued directly to the balsa sides internally? If so, it would be very weak regardless of being pinned or glassed with light weight cloth. I don't have any problems with the firewall breaking loose on my designs, but I have the firewall glued to 1/64 or 1/32 plywood that doubles all balsa sides on the inside of the nose.
On the top, it extends to the leading edge of the wing, on the bottom it extends to the back landing gear mounting plate. The sides go back to also pick-up the front bulkhead, landing gear plate, and front wing holddown plate, as well as the rear bulkhead and servo tray. In other words, a plywood box.
I don't glass the insides, or pin anything. I do fill the corners all the way around with hard 1/4" triangle stock, and glass the outside with 2 or 3 oz. cloth. The rest of the fuselage is only 1/8" balsa glassed with 3/4 oz. cloth.
This may not help much with an ARF, but if you build your own it gives an idea of what works and what is strong and light. A completely finished fuselage weighs about 6 to 7 oz. depending on balsa density.