Digital Photography School has a neat little primer on how to take good photos of fireworks:
Aperture - A common question around photographing fireworks is what aperture to use. Many people think you need a fast lens to get them but in reality it’s quite the opposite as the light that the fireworks emit is quite bright. I find that apertures in the mid to small range tend to work reasonably well and would usually shoot somewhere between f/8 to f/16.
Link - via Make