UPDATE: Instead of my FireFox Web browser, I tried using Microsoft Edge Web Browser this time, and got to AT&T's home page and was able to access my account, view my bill, etc. Using the AT&T's Web site address from Microsoft Edge, I pasted it into my FireFox browser and was able to access the Web site just like I did in my Edge browser. If you copy and paste the following Web site address into your Web browser, it should take you to AT&T's home page to log-in.
UPDATE: Instead of my FireFox Web browser, I tried using Microsoft Edge Web Browser this time, and got to AT&T's home page and was able to access my account, view my bill, etc. Using the AT&T's Web site address from Microsoft Edge, I pasted it into my FireFox browser and was able to access the Web site just like I did in my Edge browser. If you copy and paste the following Web site address into your Web browser, it should take you to AT&T's home page to log-in.
https://www.att.com/olam/loginAction.olamexecute?fromdlom=true&vhname=www.att.com:443
Here's a shortcut to the above Web site address: http://soc.att.com/26aBLQG