There is security software like Avast and Kaspersky and BitDefender and ESET that intercepts secure connections and sends their own certificate or that incorporates special web shielding features that can block content.

• https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

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.

Thank you for replying to my problem.The link you provided provides solutions for the 2015 & 2016 versions of Bitdefender.....i have the 2019 version with totally different 'protection' section

In Bitdefender 2019 you can disable specific modules.

• https://www.bitdefender.com/consumer/support/answer/13426/

BitDefender Modules -> Online Threat Prevention -> Settings -> Web Protection: Encrypted web scan

Thank you so much for your help! I followed your instructions and all is fine now with Firefox