What I had to do was hold the shutter down enough to get the lens to focus during one of the fireworks. Then continue to hold it down until the next one went up. Since it was set for a 5 second exposure, I engaged the shutter when the next firework was going up and very near or at it's apex and just about to go off.
I think I know what you mean. I went back into fireworks and check the image against the original. I think I upload the images I was dinking with. Those were the ones I had applied a sharpening filter too. I uploaded the originals, only reduced in size. Let me know if they look better now.
I'm still not seeing it in the one I posted up. However, It's probably there. The only thing I could think that would cause it is the compression format. I didn't save it using a very high compression rating in Photoshop. I think it was 4 or 5.