Fireworks are a tricky subject, still or video. If you expose for the things that aren't fireworks, the fireworks will blow out on the sensor, and if you expose so the fireworks look good, you end up with black surroundings. With stills, you set it for a long exposure and it usually does quite well and gives the fireworks long trails, but with video where you have at best a 1/30 shutter speed, you don't have the choice of holding the shutter open. Going to a bigger sensor with larger wells is usually your best bet to get proper fireworks video - Nikon D4, Canon EOS Cinema, and some of the Red cameras do the job properly.