How do I re-install without losing data

Why would you want to re-install in the first place?

So there are two "Deleted" folders

There's only one 'Deleted' folder visible in your screenshot.

Also is it a common problem that Thunderbird freezes when trying to delete e-mails from the normal Inbox.

It is a common problem for people who think it is a good idea to let their anti-virus software scan the Thunderbird profile folder. What is your anti-virus software?

"Deleted" has the figure "(*2)" after it and there is a second list of folders viewable but which cannot be copied by Win10's Snipping Tool - the screen goes blank. I hoped that a new installation might remove one of them.

Deletion of old e-mails has caused Thunderbird (38.5.1 and earlier versions) to freeze both when using Windows Defender or Avast Free AntiVirus progs. Does this help?

I hoped that a new installation might remove one of them.

It wouldn't. Mail folders are part of your Thunderbird profile, and re-installing the application will not touch the profile.

If the top level 'Deleted' folder is designated as your Trash folder you should be able to right click 'Deleted' - Empty Trash.

Deletion of old e-mails has caused Thunderbird (38.5.1 and earlier versions) to freeze both when using Windows Defender or Avast Free AntiVirus progs.

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?

Thanks Emptying the "Deleted" folder removed all the spurious folders.

I'll try the safe mode routine tomorrow but as the problem isn't always present it may be a week before I could definitely say that it was solved. Regards DonH

Opening Thunderbird in Safe Mode after selecting to disable Add-ons works, ie I do not have any trouble deleting e-mails. Opening Thunderbird disable Add-ons, "Make Changes and Restart" does not work! But as far as I can see from Thunderbird Tools, if I start Thunderbird normally ie without using Shift, I have no Add-ons or Plug-ins authorised anyway. I do hope Mozilla is working on this problem or I might change to Outlook Regards DonH

Opening Thunderbird in Safe Mode after selecting to disable Add-ons

First of all, you were supposed to start Windows in safe mode.

Opening Thunderbird in safe mode disables add-ons. That's the whole point. I don't understand what you mean with 'after selecting to disable Add-ons'.

In any case, if Thunderbird works fine in safe mode, the problem is caused by one of your add-ons.

if I start Thunderbird normally ie without using Shift, I have no Add-ons or Plug-ins authorised anyway.

I can only guess that you did disable all your add-ons in the process of starting Thunderbird in safe mode. In that case you'll need to enable them again via the Add-Ons Manager.

I do hope Mozilla is working on this problem

Not sure what problem you're talking about.

or I might change to Outlook

That is entirely up to you. Good luck.

As you can see when starting Thunderbird in Safe Mode, one option is "Disable all add ons and Continue in Safe Mode" - using that, I've had no problem in deleting e-mails. Another option is "Disable all add-ons, Make Changes and Restart" - this causes difficulties with deleting some e-mails. If I forget to use Shift when starting Thunderbird (the normal way of starting any program), a) I do not see these options b) I have problems deleting some e-mails. That is the problem I hope is being addressed by Mozilla. But also, on starting Thunderbird in this way, if I go to Tools>Add-ons, I cannot see that I have any Add-ons or Plug-ins authorised.

Safe mod also forces the default theme and turns off graphics acceleration. There is more to it that just disabling add-ons.

As you appear to have screenshots down pat. Please post one of your add-ons manger showing the extensions tab. Given Lightning is bundled with Thunderbird it would require you to have removed that for you to have no add-ons.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

As you can see when starting Thunderbird in Safe Mode, one option is "Disable all add ons and Continue in Safe Mode"

As you can see, when checking the option 'Disable all add ons', you made that permanent, even when not running in safe mode. There is no need to check any option to start Thunderbird in safe mode.

And for the 3rd time: you were supposed to start Windows in safe mode. Did you?

If I forget to use Shift when starting Thunderbird (the normal way of starting any program),

Using Shift isn't the normal way to start programs here.

When holding down the Shift key while double-clicking the Thunderbird icon starts Thunderbird in safe mode. 

Are you saying you do start Thunderbird in safe mode all the time?

I have problems deleting some e-mails. That is the problem I hope is being addressed by Mozilla.

We're still in the process to determine what's causing this behavior. A common cause for these kind of problems is anti-virus software. That's why you've been asked to start Windows in safe mode, which disables anti-virus software. But so far you've decided to ignore that.

Replies for Matt, from normal start of Windows and Thunderbird

 Application Basics

   Name: Thunderbird
   Version: 38.5.1
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20151228123010
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop.1and1.co.uk:995, SSL, passwordCleartext
     OUTGOING: smtp.1und1.de:465, SSL, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account3:
     INCOMING: account3, , (pop3) pop.ntlworld.com:110, plain, passwordCleartext
     OUTGOING: smtp.1und1.de:465, SSL, passwordCleartext, true

   account7:
     INCOMING: account7, , (pop3) pop.1and1.co.uk:995, SSL, passwordCleartext
     OUTGOING: auth.smtp.1and1.co.uk:465, SSL, passwordCleartext, true

   account8:
     INCOMING: account8, , (pop3) pop.1und1.de:995, SSL, passwordCleartext
     OUTGOING: smtp.1und1.de:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports

 Extensions

 Important Modified Preferences

   Name: Value

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Reply to Christ1 I misread your suggestion for starting Windows in Safe Mode. I did it once, learnt how to start Thunderbird in Safe Mode and did not repeat. On that occasion I had no problems deleting e-mails. Summary: I have disabled all add-ons (see images posted for Matt) yet on some occasions I still have problems deleting e-mails, the screen greys out and I get the message that Windows is looking for an answer to this problem. I will start Windows in Safe Mode for a period.

Summary: I have disabled all add-ons (see images posted for Matt)

The screenshot you posted shows plugins. While it makes sense to disable plugins in Thunderbird, that wasn't really what has been asked:

Please post one of your add-ons manger showing the extensions tab.

Plugins are 3rd party software you installed on your computer, and Thunderbird just picks them up. Plugins are not affected by safe mode.

What has been asked was a screenshot of the Add-Ons Manager extensions section.

No extensions

If the diagnostic tests eventually point to the anti virus program causing the problem, which anti virus program would you recommend trying?

DonH8888 said

If the diagnostic tests eventually point to the anti virus program causing the problem, which anti virus program would you recommend trying?

They all have their quirks and none of them as a "background" as they claim. Basically pick one that appears to do the job for you and invest the time to make it work correctly for you is the best approach.

Norons is basically the market leader, I hate it. But my bias is from more than a decade ago when it would slow a machine to a crawl. Not long after Peter Norton sold the company really.

Avast gets good reports, but has issues with Thunderbird.

McAfee has not been any good since John McAfee sold the company. But it is installed on a lot of dell computers just the same., by Dell. In recent years the front runners all appear to originate in what was once considered the soviet block.

Personally I use ESET, but I avoid "Security suites" and stick to anti virus products. Each vendor is good at something. . I am however yet to see any of them that are good at all their multi purpose security components..

Thank you both for your support. I shall remove Avast Free Virus checker Matt, experiment with other makes and only contact you again if I still have the same problem with different AV. I consider the problem solved.