What is the authentication method on the Yahoo account? Is it OAuth2 (like it should be with gmail)?

It's "Normal Password", unchanged for years. The problem showed up sometime in May, to the best of my recollection. Internet service is a bit spotty here - Fing shows 1-6 short outages per day.

Change the authentication for the incoming and outgoing to OAuth2, enable cookies in TB Options/Privacy, delete passwords in Options/Security/Passwords/Saved Passwords, restart TB, and enter your account password in the OAuth browser window when prompted.

https://support.mozilla.org/en-US/questions/1289691

Behaving well. Should I do the same for my gmail account?

Thanks a bunch!

Sure, that will work for gmail, too.

Update: the problem has recurred. Same manifestation - "Connection to server pop.mail.yahoo.com was interrupted." Strangely, the Thunderbird settings (authentication method) for yahoo pop email server have reverted (without my intervention) to Normal password, and a password was cached for yahoo mail. I deleted the cached passwords for both OAuth2 and normal password, changed the server authentication to OAuth2 and restarted TB. Same "interrupted" problem and no prompt for the account password from yahoo. gmail access works fine.

TB settings don't change spontaneously without intervention, so is there some other app that is resetting preferences, e.g. CCleaner, or are passwords being managed by an external app or some component of a security suite? If the authentication is OAuth2, the account password is not stored in TB, just the oauth token.

Hmmm. Safari sure saves passwords, but I don't see how it could be interacting with TB. I'm not using CCleaner. Maybe I did something stupid (that I can't remember) yesterday.

Be that as it may, after a morning of being "interrupted" trying to access yahoo mail, apparently the mail autoupdate from TB succeeded in getting to the yahoo server and triggering the browser for my yahoo account information. Now I have the Oath2 token in my TB cache and I can reliably Get Messages from yahoo. This is frustrating! Could it be something on the yahoo end? They have not exactly been the most reliable of services.

It could be a problem at the Yahoo end, and their POP accounts seem to have more issues than IMAP setups. Check that your antivirus isn't affecting TB passwords. The general recommendation is to add the TB profile folder as an exception or exclusion in AV settings, and disable scanning of SSL (secure) connections. A VPN, if you have one, is another common source of connection failures.

Update, a day later. Upon sfhowes' sage advice, I made the antivirus changes. One appears to have more than its share of blame. eSet, my AV program, has an option to check POP3 protocol. I disabled this option on the grounds that Yahoo mail POP servers may not adhere strictly. With all changes applied, the frequency of "Connection... interrupted" events has been very much lower, but not strictly zero. I'm attributing the rare such event to a momentary Internet outage. Thank you, sfhowes for your promptness in reply and for your pertinent advice!