If you use xrandr to switch resolutions (like, say, due to switching to an external monitor with a higher resolution), the new mail notifications from thunderbird continue to pop up at the same coordinates on the screen rather than remaining at the bottom right until you restart it.
We found that this bug has been already registered in the upstream database (https://bugzilla.mozilla.org/show_bug.cgi?id=442925) and believe that it is more appropriate to let it be resolved upstream. Red Hat will continue to track the issue in the centralized upstream bug tracker, and will review any bug fixes that become available for consideration in future updates. Thank you for the bug report.