import of mail folders by Thunderbird

My company is no longer supporting our Linux mail server (all will be handled by gmail).

Over the years I’ve run many mail clients on the Linux server: elm, alpine, squirrelMail, roundCube. My most recent client has been RoundCube.

Ideally I’d like Thunderbird to import the most current folders from RoundCube; these appear to me to be inside Maildir/ (with deeper directories like .saved-mailed, etc). But I also have Mail/ (which alpine appears to reference).

But upon adding this account to Thunderbird, some mix of folders is presented to me: not all from Maildir/ and not all from Mail/…in fact no ‘new’ Roundcube folders are presented.

Where does Thunderbird search on a linux mail server to ‘subscribe’ folders? And how can I access this location to force the subscription of the folders I actually want?