The tutorials recommend setting manual focus with reference
to something (e.g. a building) about the same distance away as the fireworks.
Ours are all open sky, so I set the manual focus on infinity.
Aperture: 8 (go higher next time--too much light!)
Shutter Speed: 1.3 seconds (sparklers around 15 seconds)
(longer allowed the wind to move the fireworks
and give the tails a funky shape)