Box shows up spontaneously in lower right hand corner of display screen and stays there even after I leave Thunderbird. It is a rectangle about 2"x5.5". When the cursor is within about 0.25" of an edge of the box it is an arrow; when inside that it is a finger/hand pointer, but does nothing when clicked. There's no message in the box, and clicking produces no response. Neither does esc or delete. Box cannot be moved from location and just sits there, blocking all views in that corner of the scfreen, no matter what program is running. It appears only when Thunderbird is running, but then won't go away until computer is turned off. AWFUL!!!