Yes, use 30-minute epoxy to attach the firewall to the fuselage. There are a couple of other things you can do to help make it stronger. First you can epoxy tri-stock in where the firewall joins the fuselage. This will create more surface area for the two parts to attach to each other. The other thing you can do is pin the firewall to the fuselage. Drill holes through the fuselage into the firewall, put 30-minute epoxy in the hole, and drive in wooden dowels (wood toothpicks work on smaller planes) into those holes. These will keep your firewall from pulling out.

Hope this helps

Ken