Try to start *Windows* in safe mode with networking enabled.

Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode

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

Does the problem go away?

The url you listed for Win 8 sends me to a site which states: "We can’t find that page"

I was able to start Win in safe mode anyway, however, when I do so I don't have an internet connection. The icon in the task bar asks me to start the Windows Security Center. When I attempt to do so, I get the attached message. This occurs whether TB is in safe mode or not.

when I do so I don't have an internet connection.

Does 'safe mode with networking enabled' ring a bell?

Duh...Sorry about that. Once I engaged Win in safe mode with networking TB appears to be working fine whether or not I have TB in safe mode. So though everything worked fine, there are x's in my task bar which indicate that I don't have an internet connection even though I actually do. See attached.

Windows safe mode disables anti-virus software. When Thunderbird works fine in Windows safe mode the culprit is almost certainly AVG. Don't let AVG scan your Thunderbird profile folder, create an exception for the profile folder in AVG. For more information about the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVG

I've known about AVG and TB conflicts for a while so I already had added TB to the exceptions and had uninstalled AVG monitoring of email. See attached. I have just run AVG's diagnostics and sent them a message about the problems that I'm having. Let me see how they respond. Thanks for your continued help.

Chatted with AVG representative who's only advice was to include TB in the exceptions folder which I had already accomplished. He said more than once that he couldn't help with the problem any more unless I purchased the paid version of AVG. At that point I switched to AVAST for antivirus management. I then contacted Dell Support and they ran numerous scans, ultimately uninstalling the reinstalling TB (which I had done previously without benefit) which seems to resolved the issue now that I'm using AVAST.

Thanks, Christ1, for all of your very timely help. Things are resolved at this point.

The Dell intervention above lasted for a day or less. Perhaps it didn't solve the problem at all and I just didn't have any unread emails for a while. Current status: Using AVAST with email monitoring turned off and the TB folder listed as exception to not be monitored. I've deleted TB profile. Restarted computer. All to no avail. TB continues to freeze "TB not responding." Dell indicated that there is nothing more they can do and that I should contact Mozilla for a resolution. Is there a way to contact Mozilla tech support directly?