From the Canon Professional Network:
"Fireworks are one of the most inspiring and photogenic, yet challenging subjects, to capture. And unfortunately, this is one of those shooting situations where fully automatic exposure and focusing may not help. However, with these tricks up your sleeve, a few accessories and a willingness to experiment, you'll capture amazing fireworks photos this Fourth of July!"Check out the full CPN Article for all the helpful tips.
Note: I'll add one more tip to the list – Combine multiple exposures in post-processing using the "Lighten" blending mode.