Might be. Or not.

What Extensions are you running please ? What Security Software are you running ?

There is security software like Avast/AVG, Kaspersky, BitDefender and ESET that intercept secure connections and send their own certificate. Sometimes McAfee. If you are running any of the above software please check their community forum for a work around to apply settings.

• https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message#w_avast-and-avg-security-products

These pages are also for the errors you may have besides relating to the above.

• https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

Plus about 6 more.....

I don't have any anti-virus / anti-spyware installed. Freshly installed Firefox, no extensions and add-ons.

What kind of error message (blue code) do you see on that error page?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

You can check if there is more detail available about the issuer of the certificate.

• click the "Advanced" button show more detail
• click the blue error text (SEC_ERROR_UNKNOWN_ISSUER) to show the certificate chain
• click "Copy text to clipboard" and paste the base64 certificate chain text in a reply

If clicking the blue error text doesn't provide the certificate chain then try these steps to inspect the certificate.

• open the Servers tab in the Certificate Manager
  • Options/Preferences -> Privacy & Security
    Certificates: View Certificates -> Servers: "Add Exception"
• paste the URL of the website (https://xxx.xxx) in it's Location field

Let Firefox retrieve the certificate -> "Get Certificate"

• click the "View" button and inspect the certificate

You can see detail like the issuer of the certificate and intermediate certificates in the Details tab.