Getting good fireworks photos is a challenge on any camera. Fortunately, Apple has continued to improve the cameras in the iPhone, and some features will help you get impressive fireworks images.
When photographing fireworks, your big issues are camera movement and focus. If you have a tripod like the Joby GorillaPod, use it. If not, steady the camera on a car roof or some other stationary object.
On to the focus issue. Apple's tap to focus feature can help you get well-focused images. You'll have to be quick, but if can tap the screen as the bright fireworks display appears, your camera will focus quickly and properly. Even better, both iOS 5 and iOS 6 have added focus lock, which most people aren't aware of. In iOS 5, tap the part of the screen you want in focus, but hold your finger there. You'll get a lock indication at the bottom of the screen. In iOS 6, do the same thing. You won't get the lock text, but the box will 'lock in' visually. Once focus lock is set, shoot away, and your iPhone camera will remember the focus setting.
Beginning with the iPhone 5, Apple has done a better job on low light performance. For fireworks, that might not make a big difference, but it will help in shooting the crowds.
The built-in Apple camera software is not your only alternative. Some of the latest 3rd party apps have extended or improved features that will help you get really good images on the 4th.
Slow Shutter Cam: This app will let you keep the shutter of your camera open, which is just the thing for capturing that fireworks display. If you have your camera steady, try 5-20 second exposures. The app is US$0.99.
iLightning Cam: This app is designed to automatically take photos of lighting strikes, but also works fine for fireworks. The $1.99 app works for video and stills.
Camera+: A standby for all iOS photos, Camera+ has stabilization, focus lock and some post processing features that will make your photos even sharper. Camera+ is $1.99.
Fast Camera: A very popular $1.99 app. It gives powerful focus lock controls, and very speedy, no-lag performance.
A couple of other thoughts. Turn your flash off. It won't help your photos, and will likely reduce the contrast. You will likely want to shoot in portrait mode, especially if you are close to the display.
Don't forget that Apple now has PhotoStream, which will let you share your photos quickly with family and friends. Wait until you have WiFi, as PhotoStream won't upload on cellular connections. PhotoStream was not available until late in 2011, so this is only the second 4th of July you can put it to work.
If it rains, you're not totally out of luck. There are several iOS apps that will create digital fireworks you can enjoy. Check out the free Fireworks Arcade and the free for the 4th, Fireworks!.