have added T to Windows Firewall allowed.

Thunderbird is not a service accepting connections from the outside. Unless you're running a 3rd-party firewall, the Windows built-in firewall is not blocking outgoing connections. So what exactly do you think you have allowed? Don't mess with firewall settings unless you know exactly what you're doing.

No other antivirus on the machine.

Since you're using Windows 10, your anti-virus s/w is Windows Defender, and it's active. Try to start Windows 10 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Should I delete the -1 -2 -3 -4 folders - and the associated msf files?

No, most likely the -4 folder (the one with the greatest no.) is the one you're actually using. You'll have to confirm this first before deleting anything. Look for 'Local Directory' in your Account Settings.

Thanks for that info. I have removed Thunderbird from the Defender app allowed list. I have not deleted any T folders as yet.

I am using Windows 11. Running it in safe mode does solve the problem of speed. (I didn't check the failure on saving passwords.}

In Task Manager there seem to be two instances of Thunderbird running. Is this a problem?

I will try again in safe mode and see if there are still two instances. Also the Antimalware Service takes a good bit of memory but I guess that is usual. See attached.

Are there any other things I could look at or do?

Many thanks Alan

I have just tried running in safe mode again; unfortunately it is not Not Responding even then. It hangs for several minutes, then displays OK but immediately goes back to Not Responding. I was hoping that in Safe mode it would be OK but seems not. Had to close it down since it was pretty much dead.

I have removed Thunderbird from the Defender app allowed list.

I don't understand what that means. The whole point is that Defender's real-time scanner will not scan the Thunderbird profile folder. So you need to add an exception (or an exclusion) for the profile folder.

I have just tried running in safe mode again;

Can you confirm you tried running Windows in safe mode?

unfortunately it is not Not Responding even then.

"not Not Responding" would indicate it works.

Many thanks for your help. I have tried various options. Yesterday running Windows in Safe Mode, Thunderbird ran quite well - acceptable speed to open and deletr messages. Just now, I run Windows 11 in safe mode and Thunderbird took ages to open . I got a message that a copy was already running. I closed it and opened it again and it took about 2 minutes to open. Now it is finally open it is running quite well. My problem is that I cannot reliably predict what will make it run slow. Earlier even with Windows in normal mode, Thunderbird was really slow to open and open and delete emails but then after 3-5 minutes it became much faster and almost as fast as it used to be.

I am sorry I cannont give a clearer clue about when it has a problem (it does often say Not Responding) suggesting something is happening but it is not clear what is going on.

Is it worth uninstalling and reinstalling Thunderbird? My profile is on a network drive.

Many thanks

Anti-virus software is most likely the culprit. 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. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile