How do you make your desktop screen smaller?

Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: command+Shift+F).

If you are in full screen mode then hover the mouse to the top to make the Navigation Toolbar and Tab bar appear.

You can click the Maximize button at the top right to leave full screen mode or right click empty space on a toolbar and use "Exit Full Screen Mode" or press F11.