Certainly use a tripod and I find that using a remote shutter release helps a ton. With the remote shutter release you can put it on bulb exposure and experiment with exposures (usually 2 to 10 seconds). I'll hit the remote as the fireworks are going up and then release it after they've fallen a good ways.
Just experiement. I usually use bulb and open the shutter when I see a firework start to go up then let it burst and fade out before closing the shutter. It's hard to overexpose when you're shooting a black sky at ISO 100, you can get good exposure anywhere from 3-30 seconds