As we celebrate the "rockets' red glare, the bombs bursting in air", the flash from fully automatic cameras in the crowd can rival the glow of the fireworks. This is the time to turn off the flash and put the camera in some sort of manual mode -- preferably full manual mode: shutter speed, aperture and focus. This particular shot was taken at 1.3 seconds exposure, f5.6 and infinity focus. The camera was on a tripod.
The general idea is that the streaks of light from the fireworks are so quick and so bright that proper exposure is essentially set by only the ISO sensitivity and the aperture. The long exposure (usually about 1/2 to 2 seconds) is necessary to catch the streak of the rocket as well as the burst.
I've put a few more fireworks photos on my website at http://hornerbuck.com/GBfireworks.aspx