Fireworks are most often if not ever detonated over crowds as they rain down debris and can also be a safety hazard if they do not extinguish before reaching the ground.
There is no way a wireless signal could "spark off all the fireworks at once" unless the fireworks were controlled by a compatible wireless system that was left completely open and triggered off of just about any type of transmission. If that were the case, anyone in the crowd could do the same thing with their mobile phones.
Finally, unless there is a TFR in the area, you do not need the FAA's permission.