In layman terms, when you take fireworks using your camera, you are using a "prolong" shutter (from 0.5s to 8s or more) for each photograph to be taken. Alot of light will be "absorb" onto the film or sensor during this "shutter lag". Therefore ISO 100 or 200 are the recommended settings. ISO 800 or above are too sensitive and might not give you the desired picture. I hope this helps.
you reminds me of shooting fireworks with my wife for the first time at the shers bridge... before we start shooting.. we prepare everything.. from gloves, covers , remote firing devices ( at that time, there is no DSLR yet, only SLR ), we even did some rehearsals...
When the first shot fires, the fireworks is so loud and glaring that we forgottened everything...ha ha...just stand there and see the fireworks.
practice makes perfect..
I normally shoot fireworks using a black glove with BULB setting. F stop about 5.6. You should get a few fireworks , say 5 or 6 , then go to the next film. Try to get some high shots and some lower shots on the same picture..the picture will be more balanced. Try to get some background also... it will be more natural...Just the fireworks itself, you cannot tell how big the firework is..When there is no fireworks, remember to cover the lens with the golves.