Display fireworks mostly are star-shells which burst at a certain height to yield colorful effects. The prediction of maximum height and the extent of spread of stars after the burst is purely guess work. Hence, a wide angle zoom lens which allows for a less tight composition is preferable which you can crop later. However, it is always great to have a big exploding fireworks completely filling the frame.
Position of camera with respect to the display:
Some people suggest that you would view the burst of the fireworks at an angle of 45° . Typically in big displays 8-12 inch diameter shells are very common and they burst on an average at a height of 1000 feet. A little trigonometry can reveal that to view a burst 1000 feet above and at angle of 45°, you have to be around 1000 feet or 300 meters away from the launch site.
Generally, a vertical frame would capture most of the details of a firework and minimize the non-informative dead space area. However, when there are horizontal elements like an illuminated bridge for example, take the picture in landscape mode.
Fireworks generate a lot of smoke and unless blown away by the wind , smoke obscures the photographs taken. Thus it is always better to be upstream to the direction of wind. However sometimes you can’t just change position according to the wind. In that case take as much photos as you can at the beginning of the display before the smoke starts accumulating and ruin your photograph. However, close up of smoke and bright light of fireworks can create some very interesting shots. So bend these rules generously.
One last chance of getting a clear good shot after the show is over:
Don't get disheartened if you were not able to take good pics for smoke, obstructing crowd, late arrival etc. reasons. For many a times, just after the show, the fireworks crew check for any unexploded fireworks remaining in the launch mortar and fire it and usually there are some. So, keep your camera ready on the tripod, wait for about 5-10 minutes after the show is over. As the smokescreen in the air gets thin or vanishes and the people start going home, comes the big bang with a perfect frame filling solitary shell burst under the backdrop of pitch dark sky.