https://support.mozilla.org/kb/cannot-receive-messages

Hi,

Everything was working absolutely fine on version 60.5.3, right up to the point that I let it take the update to 60.6.0. At that point, no new emails have come in. That was 3 days ago.

... no new emails have come in. That was 3 days ago.

That's probably not gonna change unless you follow the troubleshooting steps in the support article linked above. Please post your results.

When I removed the stored passwords for one of my accounts, then closed and restarted Thunderbird, it did not ask me for a new password. it said it was connected to that mailbox, but no mail came in.

At this point, there is no password stored for that account, but I am not being asked for the password. From this, I would assume Thunderbird is not actually trying to connect to that imap account.

While doing this test, Antivirus was suspended. I use Eset NOD32 on this machine.

Incidentally, I tried to set up a new mail account on 60.6.0 and it came back with -unable to find email settings when I did manual config, whereas on the other machine with 60.5.3 that same process finds my account and verifies the email settings.

For some reason, 60.6.0 seems not to be succeeding in making any outgoing connections.

ESET has a password manager that is a likely source of the problem if it's present on your system and controlling TB passwords.

For some reason, 60.6.0 seems not to be succeeding in making any outgoing connections.

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

I ended up taking Eset off, and it all started working again. For some reason Eset had decided to block Thunderbird.

One to watch out for - I have Eset of a different machine, and it has not done any such thing.