Why are you being prompted to create a security exception? In other words, what is the exact error message?

The dialogue box comes up asking for a security exception for pop.gmail.com. I have 6 email accounts with other providers but this is the only one where it happens (screen shot attached). All setting for this mail account are in accordance with Google's instructions regarding servers and ports. Mail retrieval is set at 10 min so if I'm away from the computer for an hour there can be as many as 6 screens to either cancel or accept - it makes no difference which option is taken.

That is a feature of Avast. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

Inspect the certificate (see the attached screenshot for instructions). Who is the issuer of the certificate you get? You may also post a screenshot of the Certificate Viewer window.

Followed link in Matt's reply and subsequent link to Avast! 24 hours ago. Followed Avast! instructions and so far this seemed to have worked. Will report back in a few days to inform results.

When your problem has been fixed can you mark the thread as 'Solved' please? Thank you.

The marked solution from Matt has been running for 4 days without error. Thanks to contributors.