You likely have pop accounts. Try disabling the setting for getting new messages for some of them.

This is working so far. I'll confirm one way or the other soon. Odd, as I had all email addresses downloading easily on the Windows 10 machine.

with Gratitude, L

No, it is not working yet. I had left two email addresses to download. bp-dcff3255-7323-44e7-b38f-bae840250204 I've now instructed one of those not to download at start-up. I will reboot the whole laptop to see if that changes anything. Anything else to try would be appreciated.

I haven't had a crash today. I don't know which bit of the above strategy has worked for me and my patience does not extend (at the moment) to experimenting to find out what I can add back in. In case it helps others, I currently have one email address which downloads at start and at regular intervals, another that does not download at start and then downloads at 6 minute intervals. There are three more addresses (rarely used) that I download manually. Thunderbird (and Windows 11) is neither in a safe mode nor a trouble shooting mode. I will wait a few days before pressing the 'solved the problem' button.

I'm still baffled concerning why I could download at start and at regular intervals for 5 addresses on a Windows 10 laptop but I can't do the same on this new Windows 11 laptop.