Try Clearing Cookies and Site Data -

Visit the site. Click on the padlock icon at the left end of the address bar. Select Clear Cookies and Site Data. In the dialog that opens, select Remove. Refresh the page and try logging in.

I had already cleared cookies and previous browsing history. I also rebooted after.

The lock was replaced by a badge today, I believe. I turned off the enhanced tracking protection and was still not able to log in. Reported, from the browser, the issue.

Any other suggestions?

Some other things to try -

Fix login issues on websites that require a username and password https://support.mozilla.org/en-US/kb/fix-login-issues-on-websites-require-passwords

Or, you can see if "Forget About This Site" will help.

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

In the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History") select "Forget About This Site"

When you revisit a 'forgotten' website then data for that website will be saved once again.

https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

You could try disabling total cookie protection. You can do that by switching to custom enhanced protection or by changing this preference. network.cookie.cookieBehavior Set it to 4.

https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq https://support.mozilla.org/en-US/kb/about-config-editor-firefox