OK - I didn't know that I might be reporting something new. Here's some more details. I leave my computer on at might (Windows XP) and it's worse in the morning when I wake up. Thunderbird's checks the email every 1 minute and it's set up to check several IMAP folders. I'm running the latest Thunderbird release as will as the latest Dovecot (not the beta versions).
In the morning it is as if it can't access dovecot at all. I get an hour glass as if it is waiting for something that's never going to respond. But if I shut down Thunderbird and restart it then everything works normal for a while.
Be sure that it's not just a memory leak in Thunderbird though. I find TB can gobble memory in some situations... you notice that it grinds the HD like crazy for example..
Ed W