Ive done a few firework displays now and found these settings work best.
ISO - 100
F8 to F11
Shutter - Bulb
RAW
white balance - auto ( as its raw you can change it to your liking later.)
Tripod ( really important to keep a steady shot)
Remote release
Also if your camera allows it turn the mirror lock function on. this will reduce camera shake.
Torch - obviously it will be a little dark so this is a handy tool.
Black paper or card - to cover the lens for multiple explosions.
my tecnique - firstly i focus my lens to infinity then once the firework launches i hold down the shutter release until the firework explodes and starts to fade away then if its a small explosion i hold the black card in front of my lens. then when the next firework goes up i remove the card and repeat the above until im happy i have enough explosions in shot. rememeber if its a large blast then you might not need too many explosions in the one shot.
we have some examples of ones i have done on our site under portfolio and events at the end this will give you an idea of what this produces.
good luck