Cannot log on to att.com with Firefox, Chromium, or Pale Moon
I can open the att.com web site, but when I enter my id and password, an error occurs. I have tried three different browsers. I have cleared cookies, started the browser in safe mode (disabling all add-ons), allowed all trackers, allowed fingerprinting, and even created a new profile. I installed a new copy of Pale Moon and made no changes to it, and it still gives the error.
The only way I have been able to sign in successfully is with the MyATT iOS app on my phone.
Here is a screenshot of the error (I x'ed out my phone number):