Is that the only site where you get a certificate error? Are you able to load and use the login page without error: https://login.live.com/login.srf When you submit that form, the site will redirect to a particular server, such as '''col125'''.mail.live''.''com It's possible that your accounts are on different servers and that one of them has a certificate Firefox finds disagreeable while the other one is fine. If the problem is with the following server mentioned by other users as a problem -- you can verify by checking the address bar of the page generating the error -- please see the linked threads for suggestions: '''bay174'''.mail.live''.''com * [https://support.mozilla.org/questions/1033421 I get the message "Untrusted Connection" just with Firefox; not with IExplorer] * [https://support.mozilla.org/questions/1019979 Hotmail untrusted connection,why?] If it's a different server, what address is it? (Just the left part, we don't need parameters after the ?)