This is what I've done before and worked quite well:
Use a tripod and a cable release (or remote) on manual bulb setting. I wouldn't close the aperture at all - as wide as possible). Set ISO to taste, try 100... and bump that up if necessary.
The first few shots of fireworks, frame your photo and get autofocus to pick up. Once autofocus has found it, swap to manual focus as not to change it.
Play around when you open and close your shutter. Sometimes you may want the trail of the firework, other times you might want the plume... have fun with it!