You don't really make sense:

Thunderbird upgraded itself from rel. 38.3 to 38.4 yesterday (Dec. 2nd 2015) and I was immediately unable to receive emails, although I could send them out with no problem.

disabled add-ons and all security software. Still no luck sending out emails.

Is the problem with sending or receiving messages now?

I restored my laptop to a day or two before Thunderbird upgraded to rel. 38.3 then turned off automatic updates.

If that means going back to a previous Windows restore point, then it must have beeb something else. Thunderbird is not covered by Windows system restore.

What's up with rel. 38.4? It's clearly buggy

It works well for many other people, so it's more likely the problem is with your setup.

Now that I have Rel. 38.3 back on my laptop, TBird is running as it should and I'm able to send and receive emails normally. My 'setup' to which you refer is identical on both releases -- I took screen shots to be sure of this, and also checked them out with my ISP. I did note, however, that Norton updated itself on the same day as Tbird, but since I disabled it when trouble-shooting 38.4 I doubt Norton is the culprit.

So we are left with the 38.4 upgrade. Nothing whatsoever has changed on my laptop in the last two weeks (at least) except for the TBird & Norton upgrades. I know this for a fact since I've not been using my laptop during that time owing to an illness in the family.

You went back to the previous version and stated not to upgrade to 38.4.0 again. There is no point to continue troubleshooting without 38.4.0 running. How do you want to proceed with this?

Well. . . I could upgrade again to 38.4 and see if the problem repeats itself. If it does, what then? I'll certainly create a restore point prior to upgrading.

Restore points are unnecessary. Simply install over top of what is currently installed with the release you want from http://releases.mozilla.org/pub/thunderbird/releases/

If 38.4.0 causes problem again, please Start *Windows'* safe mode with networking enabled - win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10 - win8 http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-safe-mode - win7 http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-7 - XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still In Windows safe mode, start thunderbird in safe mode - http://support.mozillamessaging.com/en-US/kb/safe-mode

Please post your results

I re-installed 38.4 by going to the 'About Mozilla Thunderbird' link and it worked fine. A test email to myself went and arrived perfectly, and so all is well. What happened with the auto update I have no idea. Thanks to all!