Something essential to add to you bag if you havent done so:
1- tripod
2- cable release (to use BULB
mode)
3- a thick black cloth
4- some scotch tapes
Get there early ( or go a day or so before to check out angles and lights at night), do some exposures and get a 'feel' how long the exposure is required for a 'normal night' without the fireworks.
On the night, set camera on tripod, manual
mode, shutter all the way down to BULB, plug your cable release into the camera. Sticky tape the black cloth to your lens.
Trick is, set the camera to engage exposure, start exposing your 'background' half way through and just cover your lens up, wait until you get the firework shooting up the sky and 'raise the curtain'... it should capture the entire explosion of the firework and yet keeping your 'background' well exposed.
Same trick applies if fireworks is at its heats, start the exposure and cover up the lens and wait until you see the next series of blasts coming. You can now get few blasts of firework in your exposure. You may need a smaller aperture setting (F16 or so) to accomodate the large amount of light coming through the lens.
Generally I use something like ISO 200 at F/11 and exposure is controlled by how many clusters of firework I want to get in the background.
If you can't find a black cloth, a lens cap should suffice, but this way may force you to remove the lens hood as its harder to put a lens cap on... Make sure you don't move the tripod when doing so.