"Folder in use" - Thunderbird locks empty Windows folders

[John Fossey]

I frequently save emails in a job folder (a Windows folder, using the Save As... function) to have all job related documents in one place.  When replying to a saved email, its convenient to open the saved email, hit reply and send the reply.  However, it seems that Thunderbird then locks the folder.  Any attempt to move the folder somewhere else, will result in the empty folder being left behind, with the warning "Cannot delete the folder.  Folder has been locked or is in use by another process."  The only solution is to completely shut down Thunderbird, then move or delete the folder, then restart Thunderbird, which is very annoying.  Is there any solution to this, or is it a bug?