Is there a specific error message?

Within the past couple of months, Xfinity changed many of their URLs. Maybe this is just now affecting your account. Could you:

(A) Purge Firefox's web content cache to remove possibly conflicting files. See:

How to clear the Firefox cache (only select Cached web content, don't clear all cookies and site data)

If you have a large hard drive, this might take a few minutes.

(B) Try using one of these URLs to access your Xfinity mail

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.

https://login.xfinity.com/login

I'm able to get into my email. I did see idm.xfinity.com earlier today, but all is well. Wonder if changing dns would matter?