They show the Magic Kingdom New Years Fireworks for 3 nights. Catch them on a night before NYE itself.
For NYE, watch from Epcot. The place is much larger and hasmuch larger walkways so its never really ridiculously congested like at MK. For six consecutive New Years Eves, we have spent them at Epcot in the Japan pavilion at the video dance party and people watching until the show starts. We find that the back section of the park remains less crowded. Even though we were in Japan snacking on foods, dancing, and sitting to people watch, we never officially "got our spots" for the NYE show until about 11:30. Japn has so many wide open walkways without tress blocking your view it was never a problem.
May I also suggest a room at a deluxe Epcot Resort; this way you can easily sneak out at Park closing and not deal with bus/monorail/tram lines and gridlock on the roadways. Just walk back to your room and relax....
