Not an Expert at this by any means but would try this as a starting point.
Obviously tripod mounted,Iso 100/200,f8,AWB start with exp's of 2 secs and experiment from there,try and manual focus on a firework in a specific area then take the actual shot of the next one in that area where you have already pre-focused.
If you have a shutter release??? again pre-focus on an area and with the BULB feature press the shutter just before you think it's going to go off??? then just after it's flashed then let go of the shutter release...it's all really trial and error i have tried lots of times and to be quite honest it's difficult so don't beat yourself up if everythings not pin-sharp the first time you try...
