Recently I have found that Thunderbird search has been a bit annoying. You will search the database and get results that you want, e.g. the document that Fred sent you two years ago about widgets. You can see some details about the e-mails – a tantalising glimpse of the header and the attachments but when you click you cannot access the full e-mail.
The solution appears to Rebuild the Global Database: https://support.mozilla.org/t5/Troubleshooting/Rebuilding-the-Global-Database/ta-p/15801
Which I am doing at the moment. Tools -> Activity Manager allows you to watch the process…..
Update: this works as intended. I can now find my e-mails in Thunderbird.