Although rain will not affect the performance of the firework once lit, it is essential that you keep your fireworks dry before ignition. If you do not need to lay all fireworks out beforehand then keep your fireworks indoors and set them off one by one getting a new firework out each time.
If, however you are firing a professional display and wish to lay your fireworks out beforehand then leave it as late as possible to do so. In this case we always recommend you will also need to waterproof them, you can do this by either placing them in plastic bags and removing before lighting or, our favourite and easiest method is to cling film each firework, you can then leave them in this film when placed outside. When it comes to firing simply light the fuse through the film or tear a hole an expose it. This is the best way to waterproof your pyro however give yourself adequate time to do this. You can watch a video demo of this process here.