I would say except for those locations where fireworks viewing is all but advertised as a feature of the restaurant (California Grill) it is not normal at all, and is actually inconsiderate toward your server. Your table taking as extra 20-30 minutes most likely would mean one less table that the server is able to serve that night, thus fewer tips, and the server makes less that night. If everyone did it, that's a heck of a lot of money lost.
My cynical side is also chiming in with "Hey, that's a great trick... Yeah, we're all going outside to watch the fireworks. No, no, we'll *totally* be back afterward to pay the bill... Heh heh..."
If you want to see the fireworks, schedule dinner at a time where you can see them before or after your meal.
-Rob