Is your Thunderbird email sluggish? Does the system pause when you click on an email file to read, or transfer a file to a folder or try to delete a file? The culprit is likely a corrupted inbox.msf file.

I found a solution here:

http://helpmerick.com/fix-a-slow-loading-thunderbird-email-program.htm

Basically, you shut down Thunderbird, delete the inbox.msf files, restart Thunderbird which will give you a new inbox.msf file — uncorrupted — and everything should be hunky-dory. It worked for me.

Follow the procedure in the link provided.