This morning I awoke to a Virus warning from Norton. I let it take care of the virus then switched to Thunderbird. I noticed that when I changed to a new email message, the reading pane didn't update. Figuring it was a simple crash, I restarted Thunderbird - and this time my Inbox (but not the folders beneath) was completely empty!
I did a quick Google search and found the culprit. Thunderbird stores all your email for a folder in one file. Norton found a virus in my email and it's solution was to simply quarantine the entire file (and my Inbox). Unfortunately the only solution is to tell Norton to skip my mail folders, which kinda defeats the purpose of a Virus checker, but I don't believe (afaik), that Thunderbird falls prey to viruses in email. (Unless you specifically open and run a file in an attachment. What I mean is, the mere viewing of an email should not execute any code as far as I know.)
More info can be found here.