Try to start *Windows* in safe mode with networking enabled. - Win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01

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

Does the problem go away?

Yes, started Win10 in "safe mode" with networking (option 5) and TB in safe mode and it worked.

Then tried running TB in regular mode while still in Win10 safe mode. It also continued to work fine.

Restarted Win10 in normal mode, problem comes back

Noticed that Win10 recently tried to install an update and said there was a problem when it last tried. Retried the update 1511, 10586. After the successful update, the problem is still there although if I again restart in safe mode with networking, TB works

What to try next? Looks like something is starting up in normal mode that is conflicting......

Thanks for the help...Al

Windows safe mode disables anti-virus software. So the culprit is most likely your anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it.

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

No add-ons in TB (been set that way)

Set to exclude the profile folder in McAffee. (just added this)

Set to not check email in McAfee (been that way for a long time)

No change.

As a further test, I disabled the firewall and all real time scanning for 60 minutes in McAfee. See image below. No change.

Does not appear to the the antivirus software.

What else should I look for?

Thanks...Al

Snapshot below shows TB screen and what messages look like when not working and when in Win10 safe mode.

You should ask in a McAfee forum. Also see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

Tried using the McAfee MCPR removal tool to completely remove McAfee. That fixed the problem. Now what to do about anti-virus. Should i try reloading McAfee or just use the windows defender and firewall or try some other anti-virus software?

Thanks ... Al

I suppose following the instructions you can get McAfee work with Thunderbird. But it's entirely up to you what anti-virus software to use. Do your own research and use what works best for you. In any case, I'd not run a Windows system without anti-virus s/w. Firewall has got nothing to with this. Windows comes with a built-in firewall anyway, which perfectly fits the bill.

Thanks for all of your help. From what I've read Windows Defender is a decent antivirus program. Not the best by some accounts but at least for a while I'll use it and see if I want to try another one.

This application is on another family members PC and she mainly uses it for email, and social media. Not used to browse a lot or download files, etc. So Defender may be adequate.

I've seen a lot of posts about McAfee issues with Mozilla products so probably won't reload it. Although I'm using it on my PC but don't have TB installed here.

For now, the problem is fixed and again, thanks for your help.

Regards, Al