Can you provide screenshots of the issue so we can see what is being blocked? Did you try in safe mode? Make sure to hide all personal data in the screenshot.

Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

This is the msg that comes up when I try to access my bank account:

Oops, something went wrong. If you don't think you should have received this message, you can go back and try again, or you may contact Customer Service at x-xxx-xxx-xxxx.

Reference Number: xxxxxx

IP Address: xx.xx.xxx.xxx

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

I uninstalled Firefox and re-installed it. So far, so good.