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:
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.