Comment 9 for bug 526290

From
http://ubuntuforums.org/showthread.php?p=9006611:

"Thunderbird-gnome-support" wasn't installed.
(Now I know to do this in the future on any reinstall of Thunderbird - All new installs need to have this added.)
But installing it does nothing, until you again reinstall Thunderbird3. (Luckily doing so did not overwrite or remove existing configurations). Now it works as before/intended.