Using Thunderbird 1.5.0.4 against dovecot beta9:
If a user has Thunderbird configured to move deleted messages into a Trash folder, then the user cannot delete folders in a public namespace. There is no problem deleting messages.
Thunderbird says: "The current command did not succeed. The mail server responded: Can't rename mailbox to another storage type."
All mailboxes (public and private) are maildir format.
Users can successfully delete folders from their own private areas, and the folders then appear as subfolders of the Trash folder.
Also, users can reconfigure Thunderbird to delete messages immediately instead of moving them to trash and then successfully remove public folders.
Also, users can move public folders within the public namespace. So I think the problem boils down to the inability to move a folder from a public namespace to a private one, which is what happens when Thunderbird is configured to move deleted messages to a Trash folder.
Mark Nienberg