i've shot fireworks a few times.
I really wasn't even excited about shooting them this year because i'm so psyched on getting my Canon 40D that I don't even want to shoot the XTI anymore.
I used my Tamron 17-50 f2.8 for this shoot as I was fairly close.
What I found works best is...
1) tripod is a must or your firework trails look like swimming sperm.
2) shoot in manual (M) mode so you control both fstop and shutter
3) lowest ISO you got (ISO100 for me)
4) low fstop (high number). somewhere between f11 and f22 seems right depending on shutter speed.
5) low shutter speed. 1/2 to 1 second seems good. the longer the shutter, the longer the trails, the higher the fstop (lower number).
6) manual focus. I set mine to infinity for this shoot, but when I changed zoom it blurred some of the images out. manually focusing on a few fireworks should get you sharp.