Apparently your mail files were corrupted. The problem happened already before you rebuilt the index, and those missing messages were already gone. Repairing the folder just made it visible to you.

You'd need to restore the missing messages from a recent backup, assuming you took the time to create one. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Inbox is most susceptible to corruption. It is therefore best practice not to accumulate messages in Inbox. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

Thanks for the input.

Yesterday I again lost the content of the messages, so will try your suggestion & re[ly once I discover how to do that under Norton.

regards Robert

The Norton solution did not work. Yesterday I lost all text in my Inbox again. There is an obvious problem with the programming in Thunderbird, and I'm going to be forced to migrate to Outlook. Can't tolerate the lack of support nor the lack of concern about this on-going problem.

The Norton solution did not work.

If you're still interested, you'd need to explain in detail what 'The Norton solution' is, and what you did.