Have you tried deleting your anti virus program as it is probably the cause? Usually Avast is the one mentioned in this forum, but others also insert self signed certificates into the encrypted connection so they can hack, er scan, your connection.

Most come with manual instructions on how to reduce your security so they have full access.

Thanks for replying.

I haven't tried deleting my anti-virus program. (I don't have Avast.) That seems a bit drastic, since the anti-virus is working with various other things and not just Thunderbird. It's annoying that everything seemed to be fine two days ago. So far as I can tell the anti-virus hasn't had any updates in the last day or two.

I've added the security exception, so I can now get my new emails. Yes, I know that's a risk, but probably less of a risk than deleting my anti-virus program.

I bit the bullet and am using only defender on this new device. The only thing I can say about my anti virus I have used for the past 5 or 6 years is I am not missing it.

It might be the extra ram, it might be the SSD drive but it also might be the missing anti virus. But my machine spends significantly less time waiting and reporting not responding. Mail arrives like lightning, instead of dragging in and suddenly Thunderbird has become a breeze to use where before it was struggling to even function.