To get pics of people lighting fireworks (with an SLR and off camera flash):
put your camera on a tripod, with a cable release
compose so that you'll get all the fireworks going off that you want, but leave room for the pyromaniac too; set the focus, and turn off AF
set the aperture to the correct f/stop for your flash and distance to subject (the person)
set the shutter to bulb
as the person is bending down to light the firework, trip the shutter and hold it open
when the fuse lights have the person stand up, back off, but turn and face the camera
flash them using your flash (held in your hand) with the tester button (doesn't even have to be attached to the camera) before the firework goes off
then they should run away
continue holding open the shutter until you get enough of the fireworks exploding, then release bulb
if you flash them lighting the firework, it looks like they got fried in the photo
if you have to be far away to get all of the firework exploding, have an assistant flash your subject from close up