This has going on intermittently for months. I THINK it occurs after a reboot (which I don't do very often). Thunderbird opens normally, but if I open a mail message, nothing appears on the screen. It took me a while to realize that the message window was "off the screen" and absurdly small in dimensions. The only way to see the message was to: 1) Hover over the Thunderbird icon on the taskbar and wait for the list of open windows to pop up. 2) Right-click on the icon of the tiny message window and select "Move." 3) Use the arrow keys to move the window down and to the right so that it moved onto the Desktop (note: I only have one Desktop and one monitor). After being moved, the window looks like the image I have attached. 4) Drag the window back to a normal size. If I select "maximize" instead of move, the message window maximizes properly, but since it has no active corners in this mode, I can't resize it. This behavior continues with each message window unless I close Thunderbird and restart it. From that point on, the windows open normally. Any ideas about how this might be prevented, or is it a bug?