The Mythbusters team looked at what happens when you fire a soccer ball at 50 mph backwards out of the back of a truck that is moving forward at 50 mph. The result would have come as no surprise to Galileo.
But if instead of a soccer ball, you fired a beam of light out of the back, it would still travel at the speed of light, however fast the truck was moving. (The truck can never travel at the speed of light, of course.) You would have to use the Lorentz transformations, not the Galilean ones, when you have objects traveling at close to or at the speed of light. You could use the Lorentz transformations for the soccer ball case also (since the GT are an approximation to the LT).