[Dovecot] Problems with thunderbird 3 ( messages mark unread after being read )

Hello

Since we upgrade all our users's PC with Thunderbird 3(.0.1) many of them have a problem with email that move back to unread status even after they have read them.

The problem does not occured with Thunderbird 2

Is this problem Dovecot related ? Do you have the same trouble ?

Thank you