Adding fiberglass cloth would work well as Da Rock mentions above. The problem is most likely that there is probably not enough gluing surface to make a strong enough joint of the firewall the sides of the fuselage.

Adding triangular stock to the inside of the fuselage before re-gluing should solve the problem as well. Be sure to clean the area well of old glue before adding the tri-stock and attaching the firewall. Slow set adhesive will certainly provide a stronger joint as glue has more time to penetrate the substrate. Applying adhesive to both sides of the glue joint is also a good idea to insure that you don't endup with voids in your glue joints.