the way iv;e done fireworks in the past is to use a tripod and remote shutter release using a fairly wide angle lens
usual camera settings would be something like f9 to f11 , 8 to 10 second exposure and manual focus
if you have live view it's not a bad idea to use it
how i go about setting up is i would wait for the first firework and set the focus up
an easy way is to allow the camera to auto focus on the first firework and switch the lens to manual focus without disturbing the focal point
by rights the camera should now be focused for the rest of the show ( i'm presuming your using a DSLR )
switch to live view and as the fireworks are let off i would basically press the remote shutter release button and see what comes out
keep in mind a decent fireworks display can fill an awful lot of sky it's a good idea not to be too close or you'll miss quite a bit
here's an example taken as described the settings are
24mm lens , 8 seconds shutter speed , F9 , ISO 100