the glue depends on what plane your repairing,if for instance its a built up wood plane,with slab fuse(typical box structure) and your running a nitro engine, then epoxy would be the best solution for securing your firewall you dont need to overload it just coat the contact areas and clamp untill dry.

if the firewall is mounted on a box extending out into a cowl then epoxy the wood into its place, also some triangle balsa stock in the corners really strengthens the structure.

here is a couple of examples so you have a idea of what I am talking about.
my Nextar with a OS61 2st with the firewall mounted in the fusalage typical slab sided construction.
another of my Gizmo with a OS 46 also slab const.both are epoxied in with some tri-stock for reenforcement on the back side of the firewall.

the second type is a engine box on a top flight P-47 it extends out from the fuse.
both are glued in with epoxy and are plenty strong the P-47 has a OS120 4stroke it pulls hard.
when attaching the wood former/firewall remember exess glue just adds weight not strength, coat the surfaces contacting each other and use a clamp so there is a nice tight joint,you can clean epoxy with denatured alcohol and some paper towls to wipe up the exess that may squeeze out that exess glue does nothing but add weight needlessly.
after the epoxy is dry you may chose to add some tri stock to the corners epoxy works well for this as wellbut you could use a wood glue like titebond also.