what anti virus are you using?

I am using Windows Defender

Thanks

See: https://support.mozilla.org/en-US/questions/993843

Their same issue was caused by "Browser Safeguard". Your issue may have a similar cause, that being something on your computer that is supposed to provide extra security for your web browser.

Cant seem to find anything relating to Browser safe guard on my system.

Any idea where it might be hidden

Thanks

Please attempt to add an exception on the bottom of the error page. Do not actually confirm the exception.

Then inspect the certificate - see the screenshot attached for instructions. Who is the issuer of the certificate?

Please post a screenshot of the certificate viewer window. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

This is the result I get

https://support.cdn.mozilla.net/media/uploads/images/2017-01-15-08-38-12-44a50f.png

The issuer DigiCert is legitimate. It's possible your Thunderbird certificate store got corrupted.

At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information - Profile Folder - Open Folder

This will open your profile folder in Windows Explorer.

Close Thunderbird.

In Windows Explorer, look for a file cert8.db. Delete that file. It will be re-create when Thunderbird starts the next time.

Start Thunderbird. Do you still see the exception prompt?

That fixed it

Many thanks for the help