Auto focusing on a firework is always fun. Having a guess at aperture etc. adds a little spice. The whole process is topped off by the magic automatic printing machines in the processing lab which conspire to turn everything to mud. Turn off auto focus. Focus at infinity.
Smaller apertures give more colours, but, the automatic printing process usually turns the sky grey.
Overexposure is always safer for this sort of stuff. Tripod and 10-30 sec exposures are necessary. Fireworks mostly happen on Nov. 5th. These pictures free for non commercial private perusal.