Cannot subscribe to Public mailbox

[MI]

I'm trying to set up a Public mailbox, but when I try to subscribe to it in 
Thunderbird, nothing happens: it doesn't appear as a folder, and when I go back to 
the Subscribe window, it is unchecked again.

I use postfix local dlivery, which created a Maildir when a mail arrived for this 
mailbox: /home/others/sharedmail/Maildir/{cur,new,tmp}

(sharedmail is a system user so that Postfix can deliver the mail; it doesn't have a 
login shell or a password)

To make sure it is not a simple filesystem permissions problem, I did "chmod -R a+rwX 
Maildir/"

This is my current config ( I tried to keep it short by leaving only what might be 
relevant ):

    # dovecot -n
    # 2.1.7: /etc/dovecot/dovecot.conf
    # OS: Linux 3.2.0-4-amd64 x86_64 Debian 7.9
    ...
    mail_location = maildir:~/Maildir
    namespace {
       list = yes
       location = maildir:/home/others/sharedmail/Maildir
       prefix = Public.
       separator = .
       subscriptions = no
       type = public
    }
    namespace inbox {
       inbox = yes
       location =
       mailbox Drafts {
         special_use = \Drafts
       }
       mailbox ...
         ...
       }
       prefix =
       separator = .
    }
    passdb {
       driver = pam
    }
    plugin {
       sieve = ~/.dovecot.sieve
       sieve_dir = ~/sieve
    }
    protocols = " imap"
    service auth {
       unix_listener /var/spool/postfix/private/auth {
         group = postfix
         mode = 0660
         user = postfix
       }
    }
    userdb {
       driver = passwd
    }


Also, I'm not sure about the difference between type=public and type=shared. But 
anyway, I tried both with the same result.