Variously, the following were tried:

Underpainting of the lower portion of the gun barrel white to make it appear shorter. This was tried on a number of vehicles other than the Firefly as well.

Adding a false muzzle brake made of sheetmetal at about the point where the 75mm would end to make the gun appear shorter again.

Using camoflauge netting wrapped around the barrel to break up the outline of the gun making it blend into the background better. Nature abbhores a straight line.....