Running on Windows 7, with the Classic Theme. Using a 'Solid Color' wallpaper. (Not sure how much of this is relevant). Firefox 18.0 using the default theme. Tooltips that extend past the windows client area don't get erased. They cover the window border / desktop / task bar and don't go away until something else overwrites that screen area causing a redraw.
Not sure if this is a Firefox or a Windows bug, but Firefox is the only application misbehaving this way.
Please see screenshot, I have loaded the New Tab page, and moused over to cause some tooltips to be shown. As you can see they don't get erased from outside the firefox window client area.