I had the same thing happen a couple of weeks ago. I found a post in the T-Bird help forum at mozilla.org that said to delete the .msf file. That sounded dangerous to me so I backed up the ..\Thunderbird\profiles\. folder (JIK), deleted the .msf files, restarted T-Bird and my mail was all restored.
It took a Carter to bring us a Reagan
"The principal feature of American liberalism is sanctimoniousness. By loudly denouncing all bad things — war and hunger and date rape — liberals testify to their own terrific goodness. More important, they promote themselves to membership in a self-selecting elite of those who care deeply about such things.... It's a kind of natural aristocracy, and the wonderful thing about this aristocracy is that you don't have to be brave, smart, strong or even lucky to join it, you just have to be liberal."
-- P.J. O'Rourke (1992)