Do note that with ED glass, the point of infinity focus varies with ambient temperature and may not necessarily be at the infinity marked point on the lens (btw, that is why you can turn the focus past the infinity mark on the lens)
Blurness is not due to focus of infinity. For fireworks you often need to cap your aperture down to f4 instead of wide open aperture in order to ensure sharpness, and hence long exposure times. Use manual operations for night..period. Auto exposure is totally unreliable at night unless synced up to flash.
And use a damn tripod for goodness sake, regardless on the balance compensation in the lens.