Many users have reported getting a similar error in the past two days.

Are you able to load the new "Hub" page at https://www.xfinity.com/hub/

And then you click the user-in-circle icon in the upper right corner of the page and get the sign-in form and link to check email at https://xfinityconnect.email.comcast.net/

Which step(s) lead to the error (Hub page, account panel, sign-in, email link)?

If you get an error message (not just a blank page), could you share the error text in a reply (here at https://support.mozilla.org/questions/1417754, not by email)? Please put a space before the ".com" to break the link so your post isn't diverted to spam moderation.

hard to replicate, found another link & it worked, so replaced bookmarks, but did waste a ton of time. Not sure who is at fault, changes with comcast/xfinity or Firefox change, but i am up & running again! Thanks

Since other browsers seem to get redirected to the new addresses and Firefox users have to find out the new addresses themselves, I suspect the way Xfinity set up the redirects is not compatible with Firefox. But it's just a guess.