What is your anti-virus software?

these problems persist even after switching back to 52.x

It is not recommended to downgrade Thunderbird this way. It can cause problems with the profile when the later version upgraded parts of the profile, the older version doesn't know about.

Also, profile files are stored separately from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem.

Thanks, christ1. 1.The anti-virus is Avast Internet Security. Tried deactivating it but it made no difference. The problem is there with or without anti-virus. 2. Do you suggest upgrading again to 60.x? 3. I'm still at a loss about the cause of the problem and what I can do to correct it.

Do you suggest upgrading again to 60.x?

It is always recommended to run the latest release version of Thunderbird, which is currently 60.6.1.

The Thunderbird version has got nothing to do with your problem.

The anti-virus is Avast Internet Security.

Avast tends to be problematic in connection with Thunderbird. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode - Win8 https://support.microsoft.com/en-us/help/17076/windows-8-startup-settings-safe-mode - Win7 https://support.microsoft.com/en-us/help/17419/windows-7-advanced-startup-options-safe-mode#start-computer-safe-mode=windows-7

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

Does the problem go away?

No, the problem is still there in Windows safe mode + Thunderbird safe mode (I just don't get the "not responding" message displayed but the program hangs all the same).

About the anti-virus, I've been using Avast Internet Security and Thunderbird for many years without an issue. As I wrote, the problem started as soon as I installed the first 60.x version of Thunderbird.

Thanks for your quick reply!

I just don't get the "not responding" message displayed but the program hangs all the same

That would indicate to me that Avast is at least involved.

Beyond Avast, see if this article helps. https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

O.K., looks like I have quite a bit of homework. I thought it would be simpler. Will have to pause this for a while as I'm leaving for a 4-day trip (without Thunderbird). Will resume the diagnostics as soon as I return. Again, a big thanks for your help.

Well, after a long and frustrating series of unsuccessful attempts to diagnose and remedy the problem, I finally did find a solution and it's called eM Client. It does not have the flexibility of Thunderbird, but I can at long last compose a message without stalling every few seconds with a "not responding" software. Dear Thunderbird folks, if the input of a computer-uneducated user, who uses an email client as a tool rather than an opportunity to dwelve into the program's intricate workings, is of any use to you, my hunch is that all problems started with version 60.x. Something went very wrong there and I'd suggest you give it some thought. I would be the first user to return to Thunderbird if that ever happened.

I have seen this behavior on LinuxMint 19.x (two different machines) as well as MSWin10. I can see TBird lock up my entire machine, and the only to get responsiveness back is to toggle over to a TTY (if I still can), or SSH from another machine (again, if I still can) and kill TBird. Otherwise it's hitting the power switch.

I am also having to examine other applications, as I don't see a way to get it to stop sucking up all the system resources. I have seen various hacks/edits for the preferences, none have helped. Would much prefer sticking with TB, as nothing else works quite as well.

I have suffered from the same tedious prob for 2+ years. I found a fix tonight. Read on.

From Mozilla Wiki: https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Pro...

"5. If you have gmail accounts, is problem gone after unsubscribing the "All Mail" folder?"

...with a link leading to:

QUOTE Subscriptions

Subscriptions control whether an IMAP folder is visible in the folder pane (and any lists of folders). If it is cluttered with folders you don't normally use, you might want to hide some by unsubscribing them. You won't be notified of new mail in unsubscribed folders.

You can subscribe or unsubscribe a folder by:

   Right click on the Inbox in the folder pane.
   Select Subscribe in the context menu.
   Click on the '>' next to the Inbox to expand the folder list.
   Check any folders you want to make visible (subscribe), uncheck any you want to hide (unsubscribe).
   Press the OK button. 

You can also use the Subscribe and Unsubscribe buttons in that menu. If View -> Folders is set to Unified instead of All, right click on the accounts name underneath the (unified) inbox at the top of the folder pane since that view displays the inbox folder differently. " UNQUOTE

I unsubscribed various folders in my Google account that have no use, and 5 mail accounts are now updated in less than 3 secs. My utter mistrust of all things Google is affirmed once again.