Comment 8 for bug 561323

I had this problem when launching Thunderbird from the menu, but not from the command line.
Running Thunderbird in safe-mode solved the problem, and I tracked down the bug to the Lightning extension by disabling the extensions and re-enabling them one by one.
The strange thing is that after using the calendar within Thunderbird + Lightning from the command line, the problem went away, and I am currently able to use Thunderbird with Lightning from the menu.

Very strange. I can only presume that Lightning rewrote some display settings when I used the calendar.