What is the exact error code?

Please post a screenshot. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

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

Thanks to all who tried to resolve. I use AVG as an antivirus and it updated and the problem went away. So would probably lay the blame on AVG, but since I am not a techie I can't be sure.

Good work.

When I try to log in to my AT&T account, the system hangs using Firefox

sanford.shapiro Please go to the top of any support.mozilla.org web page and use the Get Community Support link: https://support.mozilla.org/en-US/questions/new

Select the product for what the problem is (Firefox or whatever). Now select the category for the problem. At the bottom is Other.

Describe the problem in brief (one line). Then press <enter>. The website will display any prior posts that might help you. If nothing on the list can help, at the bottom press the button; None Of These Solve My Problem.

Now enter all the relative information on the problem. At the bottom of this, you can Add Images.

Under this is Troubleshooting Information. Please press the Share Data button. This will let us look at your system details. No Personal Information Is Collected.