It's possible that the bottom of the browser is behind the Windows task bar. First, if Firefox still is in full screen mode, press F11 to toggle back to regular mode. Second, use the window control menu to make sure the window is "restored" rather than maximized. Press Alt+spacebar and if Restore is not grayed out, choose Restore. Third, resize the window height using the control menu. Press Alt+spacebar, then choose Size. Press the down arrow key once, then press the up arrow key repeatedly until the bottom of the window comes into view. Press Enter to complete the resizing. From here you should be able to resize with your mouse in the usual manner. Any luck?