Check the computer time and date is approximately correct that may sometimes cause problems.

if that does not work read this article and see if you can find a more detailed error message and a solution

• "This Connection is Untrusted" error message appears - What to do

Some firewalls monitor secure (https) connections and send their own certificate instead of the website's certificate.

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• Click the link at the bottom of the error page: "I Understand the Risks"

Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".

• Click the "View..." button and inspect the certificate and check who is the issuer.

You can see more Details like intermediate certificates that are used in the Details pane.

Some examples are ESET and Bitdefender.

• ESET setup -> advanced setup -> extend web and email tree -> SSL
• SSL protocol: Do not scan SSL protocol

disable Scan SSL in the BitDefender settings.

• BitDefender -> Privacy settings -> disable Scan SSL