Sounds more like your system is a victim of malware or A/V problem. This isn't a FF caused problem.

No malware present as already scanned. This did however occur after an AVG update and now I see the solution on the avg website as follows:- This issue is most likely caused because we are adding AVG certificate into the Firefox. We would request you to try and add the AVG certificate to Firefox manually to resolve the issue. To start with, it is necessary to obtain a copy of the certificate. This can be found with Keychain Access with Keychain "login" and category "Certificates". Search for "avg". You should find "AVG trusted CA". Select it and export it from the menu bar under "File" then "Export Items...". Save it somewhere accessible.

Now, the certificate needs to be added to Firefox's certificate manager. Open Firefox's Preferences > Privacy & Security tab. Scroll down to Security > Certificates. Open "View Certificates...". Click on "Authorities" then "Import...". Import "AVG trusted CA" and trust it to "identify websites". You can select "identify mail users" if you want.

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connections and send their own certificate.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

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

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

http://kb.mozillazine.org/Error_loading_websites

https://www.bing.com/search?q=web+site+access+denied

https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean