Fireworks is an ideal time to use Manual mode.
After pre-focusing at infinity (or about that) I chose my lowest iso (100), and had been shooting between 1/3s and 2 seconds - starting with f/11.
I was actually not too far from the fireworks, and they were quite low, so I was tending to burn out. Thus I chose to shoot at f/16 for a better exposure.
I also used "Timed" exposure, so I could open/close my shutter as I wanted (each with 1 click).
This set wasn't super bright, so I was able to leave the shutter open a bit longer at 2 seconds.