Recover Deleted Messages in Thunderbird, a free add-on
Recover Deleted Messages is a simple and convenient Thunderbird add-on that will help you to recover messages you may have deleted by accident, provided you have not compacted your mail folders after the deletion.
Install Recover Deleted Messages as always, from inside the add-ons section of Thunderbird, then select the folder that contained your messages, right-click on it and select the “Recover deleted messages” option. Follow the messages in status bar.
Recover Deleted Messages can recover messages from Imap folders, if you have configured them for offline use.