[CentOS] dovecot problem -- not deleting messages from server after downloading them

I recently installed CentOS 6.0 and Dovecot 2.0.

My problem is that, when I download emails to my laptop (running 
Thunderbird), the emails are NOT deleted from my server.  They stay 
there and are downloaded again and again.  8-(

The "Leave messages on server" option in Thunderbird is not checked.

I have several laptops that used to work when I ran CentOS 5.6 / Dovecot 
1.x.  I haven't changed their configuration and they all have the same 
problem.

Here's a sample (personal data removed) of /var/log/maillog when I 
download messages (note the "del=0/487" on the 2nd line):

Aug 13 14:28:54 chasserver dovecot: pop3-login: Login: user=<centos>, 
method=PLAIN, rip=192.168.0.2, lip=192.168.0.27, mpid=18393, TLS
Aug 13 14:29:28 chasserver dovecot: pop3(centos): Disconnected: Logged 
out top=0/0, retr=487/2134628, del=0/487, size=2126323

My "dovecot -n" output is below.

Thanks,
Charlie

# dovecot -n
# 2.0.beta6 (3156315704ef): /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-71.29.1.el6.i686 i686 CentOS Linux release 6.0 (Final)
mail_location = mbox:~/mail:INBOX=/var/mail/%u
mbox_write_locks = fcntl
passdb {
   driver = pam
}
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_key = </etc/pki/dovecot/private/dovecot.pem
userdb {
   driver = passwd
}