This tends to happen when you've decided to stick to old outdated unpatched versions of software. You're running Windows 7, browsing using Firefox 52 and I'm guessing your version of Thunderbird is just as old, isn't it?

I have this problem too -

Secure connection failed

start.thunderbird.net uses an invalid security certificate. The certificate is not trusted because the issuer certificate has expired. Error code: <a id="errorCode" title="SEC_ERROR_EXPIRED_ISSUER_CERTIFICATE">SEC_ERROR_EXPIRED_ISSUER_CERTIFICATE</a>

   This could be a problem with the server's configuration, or it could be someone trying to impersonate the server.
   If you have connected to this server successfully in the past, the error may be temporary, and you can try again later.

Or you can add an exception…

When I click on add an exception nothing happens. I tried closing Thunderbird, deleting my cert8.db file and restarting Thunderbird - no change - same message. I am running Windows 10, Thunderbird version 45.4.0 Help, please ! Thanks, Sarah

Update your version of Thunderbird.