Why fiberglass the back of the firewall? That is uneccesary work. All you need to reinforce it is either some 1/2 X 1/2 or slightly smaller square basswood stock, or triangle stock. Get the corner all smeared with 30min epoxy, and place the block into place. Then, smear some more epoxy into the corners that produced (for a little added piece of mind, but not really necessary). You have a joint that will not let go. This is easiest done while building (if it's a kit). If it's an arf, I wouldn't worry about it, unless you are grossly overpowering the model.

Also, yes a lot of people use fiberglass on the center wing joint for strength. I never have as all of my 2 piece wings are arfs. In an arf, you'd have to cut the covering off, fiberglass it, then re-cover it. I don't want to put that much time into an arf. That's why I bougt an arf! Just use 30 min or longer epoxy on the wing halves and you will be just fine. I've had one wing fold up on me, and that was my fault. The plane had added weight for balancing, and was probably close to 1/2 pound overweight. It was still a fun plane to fly (trainer), but heavy. During a dive, I didn't pull back on power. When I pulled out of the dive at the bottom (probably traveling in the neighborhood of 80 plus MPH), the wing folded. I don't think fiberglass would have helped that, but maybe. So, if you are really worried about it, then go ahead. The only kit I've ever built had a one piece wing, so there was no joint. However, when I build a kit with a two piece wing, I will fiberglass it. But that's only because it's easy to do before you cover it!