I don't know what you mean by a "sent tab", because there is no tab in Thunderbird called Sent. Recover deleted e-mails from "mbox" files that have not been compacted yet. If "Compact Folders" has been used, this will not work. '''1.''' Download the add-on: [https://freeshell.de//~kaosmos/recoverDeletedMessages-0.4.xpi Recover deleted messages].  Click on that link and save the .xpi file to your computer. '''2.''' Follow the instructions in [https://mzl.la/2kYiNxP Installing an Add-on in Thunderbird] for "A slightly less ideal case", to install the add-on. '''3.''' Restart Thunderbird. '''4.''' Right-click the folder with the missing messages, and choose '''Recover deleted messages'''.