Does it still happen in Troubleshoot Mode?

Yes, still happens in troubleshoot mode.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Refreshed Firefox and aside from deleting all of my add-ons nothing else changed.

Just a guess, but is Firefox blocking any trackers on the login page? If so, you might try logging in with tracking protection disabled for the site.

Another thing to try is clearing cookies and cache for the site. I've had luck with that resolving login issues in the past.

Yes, I discovered that after reloading my extensions. This site didn’t like Enhanced Tracking Protection but gave no message asking to disable as many sites do. They must have changed something as I’ve been using this setting at this site for some years. Thanks for your reply.

You can report the site is not working from the Tracking Protection panel so it can be investigated.

See also:

• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/introducing-total-cookie-protection-standard-mode