The POP protocol

I recently upgraded to Nokia N73 and set up email on it yesterday. I was able to retrieve and view new messages (Gmail), but there's a problem here: when I try to retrieve messages from Thunderbird, these new messages are not retrieved because they are no longer new. The issue (I think) is that the 'state' -- i.e the ID of the most recent message that a POP client has retrieved -- is not stored in the client but in the mail server instead. Therefore we cannot synchronise multiple POP clients simultaneously with the contents in the server. I've read that IMAP is a superior protocol, so maybe it has this feature.