To add to the above post get yourself a black index card or make one. I set my camera on blub as well then hold the black index card in front of lens (cover your eye piece also) when you see the firework going up time the bloom. Pull the card away before the bloom then cover the lens after the bloom. Repeat a couple of times to get multiple blooms. If you do this correct you can get 3-5 blooms before it becomes over exposed. It's a way to get the finale look without having the light from the finale blow out the shot.
Also if you are doing a photo of a city scape then take a photo during blue hour (right after sunset). Buildings and the lights look their best at this time. Then just merge you firework shots with that first blue hour shot and you will have a great image. I'll try to dig up an old fireworks shot to show you.
Google it. Generally somewhere from f8-f11 at ISO 100. Barring bright lights, time is not relevant except for the ambient detail and length of streaks. But you are in the neighborhood for shutter speed. Mostly the time of day is the issue. Time is much more critical at around sunset.
I have done this a few times with RZ67 and Pentax67
I hope you have a shutter release cable
Forget setting for a specific time go with blub setting
I never had any over exp shooting at night
I think I used f-8-f11 range with ISO100 film
Wait for the launch then open shutter when piece goes off
The timing is tricky but its lots of fun