While browsing around prepping for July 4th shooting I found this article at Canon's Learning Center. From the article:
Get out of AUTO! Shooting fireworks is one specialized situation where you simply cannot expect good results with the camera’s mode dial in the full-auto green zone. Nor will autofocus work reliably. For the best and most consistent results, make sure that your exposure and lens focus are both set to Manual (and try pre-setting the lens focus to infinity.Turn your camera’s mode dial to “M” for Manual, and be sure to set the AF/MF switch on your lens to “MF”. Now, turn the focus ring until you’re set at infinity focus. Some Canon lenses have a distance scale printed on them for this purpose; the infinity symbol is a figure that looks like the number eight turned sideways. Others (like the popular EF-S 18-55mm kit lens) do not — if you don’t have a distance scale, with the lens set to “MF” and the front of the lens facing you (as if you’re staring into the lens), turn the focus ring with your fingers clockwise until it stops. Now leave it there; do not set it back to “AF” until you’re done shooting fireworks.