have a look at the following article:

How do I tell if my connection to a website is secure?

it makes sense - as a general precaution - to connect wherever possible using https. See installation for HTTPS Everywhere by Electronic Frontier Foundation

Check out why the site is untrusted (click "Technical Details to expand that section) and if this is caused by a missing intermediate certificate then see if you can install this intermediate certificate from another source.

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 of the certificate.

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

If "I Understand the Risks" is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and use "This Frame: Open Frame in New Tab".

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

• https://support.mozilla.org/kb/Secure+Connection+Failed
• https://support.mozilla.org/kb/connection-untrusted-error-message

Thank you! I will try this when I log in later & post the outcome.