For the last few days, Thunderbird has been doing something very strange and annoying almost like clockwork. Whenever it stays open too long (a full day seems to be long enough), it seems to lose track of my IMAP folders. If it then closes and reopens, it loses the entire folder structure, and so far as I can tell, all downloaded messages. If I then close and reopen it _again_, it will restore the folder structure from the IMAP server, but it will then redownload all message headers from the server. I can reproduce this reliably every time I get home from work; it's been causing annoyance on nearly a daily basis since it started.