Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Content Blocking. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature:

https://support.mozilla.org/en-US/kb/content-blocking

Ok, it does work in safe mode. I have not added any new extensions in quite a few months, so something must have changed with one of them.

I appreciate your help. I should be able to narrow down to which add-on is causing the problem.

I clicked the shield icon to the left of the website address, and I clicked to turn off enhanced tracking protection for this site. The spinner still keeps spinning. There are no warning indicators that I see.

Ok, uBlock Origin is blocking them, too. demdex.net seems to be the one causing the biggest problem. I will research it and try to decide what to do.

Uh oh, this is weird. I restarted in safe mode again, and the login page loaded. I'm a little confused, right now.

Ok, I've got it, now. I have to turn off both Firefox extended tracking for att.com and the uBlock Origin add-on. I'm marking the thread as solved, but what a pain!

That was very good work. Well done.