Space to fire makes more sense than shift to fire

Edit: Shift is a modifier key. Hold shift to go from lowercase -> capitals. In FPS games shift is walking -> running. In RTS games shift is "select one of these units -> select all of these units"

I know your game doesn't have to follow these rules, you can do whatever you want. However it's worth thinking about what "language" each key on the keyboard is speaking when you want to use it.