Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, copy files like Sent (mbox files) into Mail/Local Folders, renaming them if necessary, restart TB, find Sent under Local Folders in the Folder Pane.

Strangely enough, Thunderbird doesn't seem to have built-in import/export capabilities for individual mailboxes. That functionality must be added to Thunderbird by installing an add-on such as ImportExportTools.

The way that Thunderbird stores and manages mail in mbox files within the profile folder, however, makes it possible to import mail stored in an mbox file by merely moving it (with no filename extension) to the desired place within the mail hierarchy in the profile folder, while Thunderbird isn't running. The next time it is launched, Thunderbird will automatically recognise the mbox file, create the associated .msf index file, and show it as a folder at the corresponding place within the mail hierarchy in the folder pane, with all the messages in it. This is what sfhowes suggested.

The manual file moving approach has a caveat, though, namely that there is not a single mbox format, but rather a set of file formats collectively known as mbox, which in some cases may make it preferable to use an import tool to ensure the resulting mbox file will be in the exact mbox format expected by Thunderbird, but if the mbox files you want to import were created by Thunderbird to begin with, that's obviously not an issue.

Thanks to you both for the help.