Here's a late reply to this. I was trying to get my full screen FF to scale down too. Here 's what I did...
Go to menu WINDOW > ZOOM
This should stretch to fill the screen.
Do WINDOW > ZOOM again.
Resize the window to what you want by using the bottom-right-corner sizer.
Quit and restart FF to see if it worked.
It worked (mac os 10) thanks a lot!!!