I use Thunderbird 3.0.4 on a Macbook Pro running OS X 10.10.5, but my problem started several versions ago. Sometimes when I use Thunderbird, a small white box appears on the screen, with a "Get Mail - W" header, see screenshot. Single or double clicking the box does nothing at all. Control-clicking it has no effect and neither does the Escape key. I cannot click and drag it anywhere. It just remains on top of all other programs. The only way to get rid of it is to close Thunderbird. It will be gone when i reopen the program. Please help.