Comment 2 for bug 593459

According to http://ubuntuforums.org/showthread.php?t=1442764 this is a problem of thunderbird itselves, and the following package will provide a workaround: https://addons.mozilla.org/en-US/thunderbird/addon/12523.

I did not read the source code of this addon, but it seems like the add-on uses gnome's gnome-open program which tells gnome to act as if someone double-clicked on the file in nautilus.