There have been issues reported with the idm.xfinity.com server and it appears that Comcast/Xfinity has forgotten to update/renew the certificate for this domain. You can contact Comcast and point them to the Qualys SSL Report for idm.xfinity.com. *https://www.ssllabs.com/ssltest/analyze.html?d=idm.xfinity.com *https://www.ssllabs.com/ssltest/analyze.html?d=idm.xfinity.com&s=96.99.230.105 *https://www.ssllabs.com/ssltest/analyze.html?d=idm.xfinity.com&s=2001%3a558%3afeed%3adc%3a0%3a0%3a0%3a18 Try to login via this page: *https://login.xfinity.com/login See a thread like this one *[/questions/1420928] unable to sign into Xfinity for email More: *https://support.mozilla.org/en-US/search/?w=2&product=firefox&q=field%3Acontent%3A%7Bcomcast+email%7D+range%3Aquestion_created%3Agte%3Anow-30d%2Fd