Best generally to start shooting wide, afterall, you can always crop later whereas if you are zoomed in, you can't 'recover' what you've lost. But sometimes you think that 50mm should be fine only to find it's not, so have a wider lens to hand should you need it to facilitate a quick change if your initial set up isn't appropriate. And wear a head torch as you are likely to be in a dark field or park when you are trying to change your gear.
The reason why Bulb is so useful is that the firework action is unpredictable and usually happens in bursts with small pauses in between so it is best if you can take full control. After a set of fireworks go off and have done their thing, you can let go of the trigger (so closing the shutter) and then click again for the next set. With the fireworks so bright, if you leave the shutter open too long, you can over saturate the picture with light and the fireworks lose their definition as they stack up onto each other resulting in a single blown out explosion. Bulb allows you to stop the action when you think it best to do so, usually after a spectacular firework has done its thing.