I had the same problem during my brief flirtation with Thunderbird. I came to the conclusion that there is something inherently unstable about it as it kept losing mails, settings, and addresses.
I went back to Windows Live Mail for Win 7 on my laptop - don't like it as much as Outlook Express but that doesn't work with W7, and I've stayed with Outlook Express on XP on my PC. I'm dreading buying a new PC and having to start all over again.