I agree, you need to get a Nifty 50, or something along those lines. I personally have an old XS, and I can tell you that ISO 1600 and F1.8 is not enough on HM. You can get some semi decent shots on POTC, but the higher the ISO here the better.
You can check the quality of your manual focus by using the zoom buttons and looking at your display. Everything looks good when your using a 3" inch screen, until you zoom in.
Dont delete your pictures until you have them downloaded on a laptop.
Bring a backup battery, using a remote trigger and tripod for fireworks, the photography section of the Disboards has a primer on how to shoot fireworks. But most of all, find a good spot and get there at least an hour to hour and a half early.
Matt