If you want to add strength to the firewall area, using thinned epoxy "in the corners" won't add that much strength. Thinned epoxy is used more for fuel proofing than adding strength. If you want to add strength to the firewall area get some triangular balsa stock. Cut 2 pieces and epoxy them into the area where the firewall meets the fuselage, on the fuel tank side of the firewall. You can also use some 3/16" dowel to pin the firewall. Drill holes through the fuselage into the firewall, cut 3/16" dowel pieces (longer than the hole), add a couple of drops of Elmer's glue into the hole, tap the dowel down into the hole, and after the glue has dried saw the dowel off flush with the fuselage. See the attached pictures for ideas on strengthen the firewall.

IMHO using thinned epoxy to strengthen joints will add more weight and won't stringed the joint that much, at least not compared to the added weight. If you want to make joints stronger use balsa tri-stock and some Elmer's carpenters glue. if you use the glue very sparingly and apply a very very thin coat of glue on the tri-stock will give you a very strong joint with not too much weight gain.