Several users in recent days reported problems signing in to their AT&T mail (hosted by Yahoo). They are getting an error screen from AT&T but it sounds like your issue is somewhat different. In case it might be relevant, here's a thread on that issue: https://support.mozilla.org/questions/1490607 (unsolved).

Firefox's "Total Cookie Protection" feature limits how sites can use "third party" or "cross-site" cookies, meaning cookies from different websites than the one you see in the address bar. Some sites pass you back and forth between different servers during login because they have their services spread out over a number of different servers. In those cases, you might be able to submit a form successfully on one page and then the next page says you are not logged in. Frustrating!

You can make exceptions to this feature when needed (for each server that you visit as part of the process of using this company's services). More info in the following articles:

• Introducing Total Cookie Protection in Standard Mode
• Total Cookie Protection and website breakage FAQ

Any difference?

I tried to disable Total Cookie Protection but there is no shield button to click on as I am out of the website immediately.

This problem does not exist with other browsers. So the problem is with Firefox.

Try using a private window (Ctrl+Shift+P).

You can also add an exception using the Manage Exceptions button on the Settings page, Privacy & Security panel, in the Enhanced Tracking Protection section.