I have the same provider https://login.xfinity.com/login and no issues. see screenshot What happens with Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

You can also try the option to Forget About The Site, but make sure you know the login information. https://support.mozilla.org/en-US/kb/remove-websites-address-bar-suggestions

Thanks for your help. I used the URL you use, and it worked in Firefox.

Using www.xfinity.com on Firefox didn't work. It doe work on my 2 other browsers, Bing and Chrome. Firefox seems more finicky for the correct log in.

That is strange because that xfinity.com works for me. see screenshot If you have no further questions mark it as resolved and have a good day.