[Dovecot] tags disappear in Thunderbird

We have -

Dovecot 1.0.15 (the default on Debian Lenny stable) Thunderbird 3.1.4

In our installation the Inbox is a mbox and the other folders are maildir.

When we tag messages in Inbox, the tags disappear after a while. In the other folders they stay.

I found two possible solutions suggested -

1. Set mbox_lazy_writes=no

2. Convert the Inbox to maildir

What would be the best approach in your opinion?

Would suggest not following the Debian Lenny stable version (1.0.15), but compiling the current stable Dovecot?

Thank you very much, Iv