Hi, does the error page have an "Advanced" button? If so, could you click that to expand the technical details, and then copy/paste that information in a reply (here on the forum, not by email).

More generally...

Within the past couple of months, Xfinity changed many of their URLs. Maybe this is just now affecting your account. Did Xfinity support ask you to try using one of these URLs:

Direct email link: https://xfinityconnect.email.comcast.net/

New hub page: https://www.xfinity.com/hub/ -- toward the upper right, there's an account button (looks like a little head and shoulders in a circle) that opens a panel to sign in. Any luck using the "Check Email" link on that account panel?

Hi, are you having a connection problem on a page with an address at idm.xfinity.com -- revoked certificate? Unfortunately, this is a problem on the server where it looks like someone forgot to update a certificate.

Firefox may be the last browser that does a real-time check for whether a site certificate has been revoked, so it is the first affected by this kind of problem. Firefox doesn't have a way to exempt one site from this check; it is all sites or no sites. For that reason, if you are okay with checking your email in another browser for a while, that probably is safest, rather than me describing how to turn off certificate revocation checking globally.

It's what I'm getting. see screenshot