Problem configuring POP3 for Thunderbird 68.7.0 with 2FA and Gmail app password
I re-installed an existing Thunderbird client (Mac) when version 68.0 was released because the existing folders were present but not displayed in the left pane. The client was set to check messages via POP3 (SSL/TLS) from a Gmail account using 2FA and an app password. I'd like to keep the message store from what's been saved over the past few years. I've configured Thunderbird and Gmail with 2FA (but not an app password) to check that it works. I have no such luck with POP3. Sometimes it looks like a password error. Other times Thunderbird can't find the client configuration data. If I can import the existing mail to Gmail via IMAP, I can join the modern age and use IMAP, but that's a different project.
Opaite Mbohovái (1)
I'm a bit confused by your statement about folders not being displayed. A POP setup will only download the Inbox, while IMAP can display subscribed folders other than Inbox. No matter which you choose, the recommended gmail setup with the latest TB 68.7 is to apply OAuth2 authentication. Delete the existing passwords in Preferences/Security/Passwords/Saved Passwords, restart TB, and enter the regular account password in the OAuth browser window when prompted.
http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite