Are you online login to the site? Firefox isn't not a email program unless your using Thunderbird. If Thunderbird then you need to get the login settings from att. net on how to setup Thunderbird to receive and send out email. Or try another time as that could be a email server connection issue.

I do not use thunderbird

I just tried and clicked to open that link on Chrome and it says it doesn't exists? So where are you getting the link from during the login?

This is the link that I use: https://oidc.idp.clogin.att.com/mga/sps/oauth/oauth20/authorize?nonce=okzNLRcED9&redirect_uri=https%3A%2F%2Fatt-yahoo.att.net%2Fisam%2Fsps%2Foidc%2Frp%2FATT-HBO-RP%2Fredirect%2FYahoo&response_type=id_token&response_mode=form_post&state=CRA2c2P7jm&scope=openid&client_id=m40842

So the error shows up after you enter your login is that correct? The only other way is for another att user on here which I hope there is to login with that same link and tell the rest of us what happens when they use the lastest Firefox to login. I clicked the link and it gives me the login screen but without a account that's the extent that I can see it works in chrome.