Sorry, that is TB 2.0.0.6
Select on that sent folder , right click on it and select Delete .
If it fails to work , close Thunderbird .
Goto Thunderbird profile folder
["http://kb.mozillazine.org/Profile_folder_-_Thunderbird"]
Backup your profile folder .
Open Mail folder . Open Local Folders .
Delete/move _empty_ Sent file , 'Sent.sbd' folder and all .msf files .
Start Thunderbird .
Gary
http://kb.mozillazine.org/Phantom_folders
http://kb.mozillazine.org/Profile_Manager
Gary