Using bulb mode for fireworks, u can use the black card method to vary exposures. This is done by holding a black piece of card in front of your lens and apply the removing and placing it back to expose the parts that u want.
Bulb mode = varying shutter speed, depending on how long u press the shutter. if u press for 2 seconds, shutter speed will be 2 seconds. if u press 10 sec, it will be 10 sec. so, for fireworks, use a tripod and use a cable remote if possible, else, use the shutter button on the camera. if u wan the trail of the firework, depress the shutter button when u see the thing flying up and hold it till the fireworks gone. all things that happen within the time the shutter is open will be captured in the camera
Firstly, you need to know how bulb mode works. It is mainly for long exposure in a very low lighting environment. The camera will start to exposure when you press the shutter until you release it. So you observe the fireworks pattern before it fires off (press the shutter and hold it) until it end (release it). Then you will capture the whole fireworks pattern.
Of course you should use a shutter release cable to prevent any vibration to your camera setup.
You can try bulb mode test at home. Turn off all the lighting, use a torch light shinning around the walls. Use the bulb mode to see how the camera capture the torch lighting effect.