Update--I've been using the URL (https://login.yahoo.com/) mentioned above successfully since posting it on 16 May (almost one month). Yesterday Mozilla updated Firefox on my laptop. Now I cannot log into my sbcglobal email account using either this URL or the "old" URL. Cannot log into currently.att.yahoo.com/ either. Using the cell phone hot spot does not work for me either.
Today, I can log into my sbcglobal email using DuckDuckGo today (who knows about tomorrow).
I'm not normally a conspiracy theory person, but it seems to me that the various entities are not playing well with each other and I'm not sure why. I refuse to only use products by one "entity"--hence, I don't use Microsoft's Edge browser. I don't use Outlook, so don't know if there is problem there. Maybe there is a protocol issue. I have the Yahoo email app on my iPhone and I have been able to read my emails there. The problem is that I'm a touch typist and hate trying to write anything on my phone!