Erm just my 5cents i think which is worth alot
1. Fireworks highlights are way overexposed
-think your shuttle speed are too long
-think your f/ stop value too low
Ways on improving?
B4 e event starts, test your camera, how?
1.Firstly u came to shoot fireworks so do your settings,
2.Shot at objects which have light. See if the results are good? (under exposed/over exposed)
3.If just right, estimate if your settings will harm your fireworks pics later. If u think that the fireworks is going to be too bright, step doen the shuttle speed 1x. For in your case i think step down f/ value 2 steps if it is at max if not 1 step.
4.As everyone says, use lowest ISO for the fireworks.
hope this can help u abit.
for me my fav settings are
fountian fireworks (on ground & repeatadly same motion)
ISO 200, 3", f22,
Sky High Fireworks
ISO200, 8-10" F22
if u wont like to see overlaps i think 6" will b best.