This happens when an e-mail file that gets built each time you open e-mail gets corrupted. It is: {inbox.msf}. Here's how to fix it:

1. Close Thunderbird.
2. Click Start --> Search (or Find --> Files and Folders in Windows 98/Me).
3. Type: inbox.msf in the search box.
4. Make sure you have the box checked to search hidden and system files.
5. When it finds it, click one time on in and then delete it.
6. Start Thunderbird again and a new, uncorrupted inbox.msf file will be created.

This may take a little while the first time. To test all is well, close Thunderbird and re-open it and it should open quickly