Getting the focal length right for your fireworks pictures is not easy. I generally want the entire rocket blast contained within the frame. What makes focal length selection difficult is that these blast range dramatically in size.
I usually select a focal length that will contain about 80-90% of the blasts. I crop the smaller explosions and determine if the ones that exceed the frame result in an interesting picture or not (the "nots" are deleted of course).
This firework blast fit perfectly in the frame. With the high resolution Canon EOS 5D II behind the capture, I have lots of freedom to crop into the picture. I can of course use the picture as-is, or, with completely black borders, I can extend any edge of the frame by adding more black.