I have not attended a fireworks display in many years but this is how I did it and came up with some really great images. Granted, this was done on film and not digital but things are really no different and you don't have to worry about reciprocity effect like you did with film. I cut a card about 5x7 and sprayed one side flat black. I would place the camera on a tripod and point it where I expected the fireworks to be. I then closed down to around f/11 (using ASA 160 film) and placed the card over the lens and locked the shutter open. Once I heard the fireworks go off, I removed the card from in front of the lens and left it off until the fireworks had gone off and then placed it back over the lens. I repeated this once or twice more. This is in essence doing a double (or triple) exposure without having to go through the steps to do one in the camera.