Benadryl, or diphenhydramine hcl, doesn't really help much for stress. It's true that it can make dogs sleepy, and if given soon enough before the fireworks they may sleep through them. The problem is that some dogs have the opposite reaction and become hyperactive. Hyperactive and stressed will just make for a bad evening for everyone.
It is best to talk to your vet in the days before the fireworks start. A prescription is likely to be a better option for your pet. Talking to your vet first will also help to make sure you get the dosing right for your dog.