I used to get this when I used a different email program. The problem was the email that was being downloaded. I always put it down to some rogue programming or a corruption in the email itself rather than the program I was using. I would go to your account online using a browser and look at the email that's causing the problem. First, deal with anything in it that you need to do. Then delete it. When you now try to use Thunderbird, do you get the same problem?