Starting a new thread for your problem is the right thing to do. Posting to an existing thread usually is a bad idea, as problems tend to be different for different users, even though they may look like the same.

You'll need to provide more details about your "confirm security exception" problem. Please post a screenshot of the error message. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

OK. Whenever I start Thunderbird, I get a window pop up that asks me to confirm a security exception - see the screenshot. If I do so, it pops up again, and I have to press the "confirm" button again. After doing this half a dozen or so times, it is happy for the rest of that Thunderbird session. This is therefore an irritant rather than something that stops me using email. However, the fact that I get the window indicates that something is wrong - as someone said in response to a previous answer to a similar (or identical) problem. I am using Windows 7, Eset anti-virus software, and my ISP is Falcoda. Previous threads on this seem to indicate that I should configure Eset to not inspect email traffic. Is this, in fact, the preferred solution - or would that lay me open to emails introducing viruses or malware? https://support.cdn.mozilla.net/media/uploads/images/2017-01-21-10-10-04-af316e.png

Without having seen the details of the cert you get, disabling the interception of secure connections as described in ESET’s support article would be a good idea. https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER#w_eset

The effect is that ESET will not be able anymore to scan your incoming and outgoing messages. That will not put you at risk in any way. I'd say it will increase your security, as ESET also won't be able any more to spy on your password.

Some more information about why scanning email messages is not needed. http://thundercloud.net/infoave/tutorials/email-scanning/

Thank you very much for your reply - which I guess has solved the problem as fully as it can be solved. I got onto Eset as recommended and disabled the automatic SSL filtering; pressed OK, and then rebooted and started Thunderbird. To my surprise, I again got the "Confirm Security exception" window. I don't understand how this happened if Eset was not monitoring it. However, I clicked "confirm", 3 times, I think, before it stopped appearing. Then I rebooted again and restarted Thunderbird again. THIS time, the "Confirm security exception" window did NOT appear, so, at last the problem seems to be fixed. I guess what happened is that pressing the "Confirm" button has now been effective in permanently storing the exception. I don't fully understand what's going on - but, thanks very much again for your help.

I guess what happened is that pressing the "Confirm" button has now been effective in permanently storing the exception.

You should not need an exception any more now, and you can delete any permanent exceptions you created.