I was shooting fireworks a few weeks ago, and I used manual, because setting the speed and aperture have quite different effects in this case. (I.e. the automatic setting, which assumes that both control the brightness/exposure of the final result, simply does not work.)

Fireworks are essentially moving particles of light. The speed (exposure time) you use will not influence the brightness of the streaks on the image---it is the movement speed of the particles that determines how long a given sensor pixel will be exposed to their light. The speed setting of your camera will only determine the length of the light streaks. (Of course if you use a very long exposure, the smoke in the background will be more visible.)

To control the brightness (exposure), you can use the aperture setting.

I did a few experiments to find out what aperture will give the correct exposure, and then played with the speed to get different effects. I needed to adjust the aperture a bit sometimes for different types of fireworks, but you'll quickly get a feel for this after a few test shots.