I have noticed this too, just the other day, in fact. Renaming the folder (a subfolder of "Local Folders\Personal Folders" before deleting it is a workaround.
Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.1.9) Gecko/20100317 Thunderbird/3.0.4
I have noticed this too, just the other day, in fact. Renaming the folder (a subfolder of "Local Folders\Personal Folders" before deleting it is a workaround.
Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.1.9) Gecko/20100317 Thunderbird/3.0.4