[Dovecot] can not get t'bird to create folder

[sean darcy]

dovecot-2.0.16 on Fedora 15.

thunderbird-8.0

I can access dovecot from thunderbird, imap maildir. But when I try to 
create a new folder, nothing happens.

I'm not using LAYOUT=fs, so I have added, tb-extra-mailbox-sep, which 
didn't seem relevant anyhow.

Any help appreciated.

sean