Try the following:
1. Press F11 to switch to the real Full screen
2. Press F11 again to switch to a normal maximized window
3. Switch from maximzed to normal Window
You might get a window that is larger then your screen resolution:
4. Press ALT+SPACE, select Move and move the window so that you can reach the border
5. Resize the window so that it fits on your screen
6. Restart Firefox and hopefully everything works now
Try the following:
1. Press F11 to switch to the real Full screen
2. Press F11 again to switch to a normal maximized window
3. Switch from maximzed to normal Window
You might get a window that is larger then your screen resolution:
4. Press ALT+SPACE, select Move and move the window so that you can reach the border
5. Resize the window so that it fits on your screen
6. Restart Firefox and hopefully everything works now