I just bought a new Kodak easyshare zoom digital camera & it actually has a special setting for fireworks. I have'nt tried that feature out yet but so far I love it. It's pretty small. I have a Kodak digital that's 4 years old & it's still a good camera, it's only 3 megapixels & the new one is 8. It was only $99 at Walmart.
Check out Panasonic TZ5. It has 10X zoom and is getting great reviews over on the DPreview board. In fact, there are some fireworks pictures posted over there that were taken with the TZ5.
To get the best picture you should use a tripod and I don't know if WDW allows them or not.
Google "How to take pictures of Fireworks", you will get a lot of good advice.
The good news is that with a dig camera you can see your results right away when I used to do this with my SLR I had to wait 4 or 5 days for my pictures.