Fiberglassing the firewalll is pretty much making more work for yourself that you don't need. For most planes a good tight fit, 30 minute epoxy, and triangle stock will be more than enough to make a strong firewall. Use the triangle stock on the backside of the firewall where the firewall meets the fuselage sides. (maybe we can talk Minnflyer to do a one of his great graphics showing this). If you do this you shouldn't need anything else to secure your firewall in for most planes

Hope this helps.