My theory on the Firefly being left in the hanger: because is a slow compared to other tanks, and the optics, gun, and armor offer little over a M4/76. At longer ranges (2000+ yards), the Firefly holds a true advantage, but in AH those ranges are not as common as the under 1400 yards ranges.
I would love to use the firefly for 2k shots, but you rarely ever get a chance to fire over 1400 unless the map has hills