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

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

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

None of the referenced software, or any other security software, is installed or running on my Mac. The only thing that has changed is the new version of Firefox. These sites worked up until this "upgrade." The sites work on Safari, formerly my backup soon to be my default browser. This is not the first time a Firefox upgrade caused significant issues.

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}acking-protection-firefox-desktop

Opening in Safe Mode the issues weren't there. When I restarted Firefox with Safe Mode this issues had also been resolved. Thank you.

Glad to help. Safe Surfing.