[Dovecot] Thunderbird and shared mailboxes

I'm setting up shared mailboxes as in http://wiki.dovecot.org/SharedMailboxes/Shared, and have set ACLs as described on that page to share one folder of user A's mailbox with User B.

However, I'm struggling to get Thunderbird for userb@example.com to read the shared folder of usera@example.com. Has anyone done this?

Thunderbird 3.0, Dovecot 1.2.11

Thanks, Keith