Windows 7, Firefox in full screen mode (don't know if this is important), Firefox option "tabs on top" active. After having used the Windows "Snipping Tool" (a very convenient alternative for "print screen"), often Windows and Firefox are both trying to own the top bar of the Firefox window: * In some cases only the Firefox tabs will be visible, but not the standard Windows minimize/restore/close buttons on the right * In other cases the tabs will be invisible (except for the lowest pixels) and Windows will "own" the top bar. Sometimes this issue will disappear with proper clicking or with a minimise/maximise from the Windows taskbar, but generally the issue will pop up at irregular moments until the user is logged off.