My mastiff X is not afraid of thunder. When I first got him, I would walk him during thunder storms to get him used to the noises. It doesn't phase him anymore, usually the same with fireworks. However... He is scared of just about every other darn thing imaginable :| Kids, the dark, masks, guns... He even gets spooked if you walk up to him with your hands in your pockets and he's never met you before, because he can't see your hands!! What a freak.