What is your anti-virus software?

My anti-virus software is 'ESET'

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 - Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode - Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-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?

I followed instructions for Win 10. Everything was straight forward until the Restart button. Everything restarted in expanded mode with a window saying it could go no further, but try a different application, and that's where I could go no further.

Ask in a Microsoft forum about starting Win10 in safe mode.

I'll do that. I've found additional problems with TB which may have some bearing on the 'not responding'. The last reply from Christ1 on 22/8/16 arrived in TB with 5 other emails. I clicked on the email below it and Christ1's message disappeared completely. and I can't find it anywhere (except in this forum). This has happened before but as the mails looked like Junk I wasn't to concerned. More problems are the Delete and Junk buttons which don't work despite having been correctly defined in the Settings. Because I can't use the delete button (or sometimes even the delete key on my keyboard), I've been dragging the unwanted messages from the Inbox list to either the Bin or Trash labels. Sometimes it works, others times it hangs the program or they just disappear, never to be seen again.

Is it possible to reinstall an earlier version of TB as a means of testing, or would it be better to completely delete TB and all associated files (except messages) and start all over again? Or is that like using a sledge hammer to crack a nut?

Chances are your problems are caused by anti-virus software. Starting Windows in safe mode disables anti-virus software to determine whether it is the culprit.

Re-installing the application won't do you any good, as Thunderbird separates the program installation and the profile with your data. Your problem is with files in your profile.

Going back to an earlier version is not recommended, as it would expose you to security vulnerabilities. And it wouldn't address the problem with anti-virus software either.

I started PC in Safe mode and it was so safe nothing much would work because most actions needed access to Gmail server. Next I tried Safe Mode with Networking. The problem still existed. Next I disconnected my anti-virus software. No difference. Then I tried Microsoft Forum. A Microsoft person suggested trying installing TB in Compatible Mode. I presume that is another term for Safe Mode. This is where I'm stuck because you said reinstalling was not a good idea. Would downloading over my current version, in both Safe Mode and with anti-virus switched off help? Or any other suggestions please. Thanks.

Next I tried Safe Mode with Networking. The problem still existed.

Did you also start Thunderbird in safe mode as instructed above? https://support.mozilla.org/en-US/questions/1133586#answer-904026

The question really is whether you still saw the 'not responding' issue with both, Windows and Thunderbird started in safe mode.

Any other issues you raised about 'Delete and Junk buttons' are a different story and wouldn't disappear in safe mode.

Yes, I started in Windows Safe Mode and Safe mode with Networking. This message was displayed "Ths App can't open using the Built-In Administration Account. Try a different Account".

I don't understand that. I don't use a password to access Windows if that has anything to do with it.

Anyway I opened TB in Windows "Safe Mode with Networking" but although I got the message "Failed to connect to Server" I was able to drag messages between Labels and Accounts without a problem, but got several new messages duplicated, although TB wasn't supposed to be connected to the Server.

Next I tried Safe Mode with Networking. The problem still existed.

How do you think anyone could have guessed what happened with that kind of response?

I started in Windows Safe Mode

You were not instructed to start Windows Safe Mode (without Networking). Why you tried that anyway I don't know.

"Ths App can't open using the Built-In Administration Account. Try a different Account".

While it should be possible to start Thunderbird with an Administrator account, this is a bad idea regardless. Again, I don't know why you did this. And it shouldn't be related to the problem at hand.

I opened TB in Windows "Safe Mode with Networking" but ... I got the message "Failed to connect to Server"

Are you using WLAN? if so try with a wired connection.

although I got the message "Failed to connect to Server" I was able to drag messages between Labels and Accounts

Is this a Gmail account? What account type - POP or IMAP? What other accounts?

Did you see the 'not responding' issue while being in safe mode?

For the 3rd time, did you also start Thunderbird in safe mode?

Please be precise when responding. Thank you.

Obviously my non-technical mind has answered in the wrong way. I started Wndows in Safe Mode with Networking where the message is displayed regarding a Built In Administration Account. OK was the only option which took me into Windows in Safe Mode. I then ran TB but although I got the message "Failed to connect to Server" I was able to drag messages between Labels and Accounts without a problem, but several new messages arrived duplicated, although TB said it wasn't connected to the Server. Anyway the problem expands because Dreamweaver SC3 is now occasionally displaying the "not responding" message. No specific action seems to cause this, so this problem is bigger than just TB. Looks like I'll have to search through Windows forums. Thanks for your help.

Also see https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems