You provide no email provider so anyone reading can verify what is happening when you use that email provider site. Also posting screenshot errors helps other verify or replicate the issue.

Hi, is it Xfinity?

Many users have reported a connection problem on a page with an address at idm.xfinity.com -- their various other servers work fine. Unfortunately, this is a problem on the server (or in their Content Distribution Network) 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.

A couple other things you could look at:

(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?