hi, unfortunately MS has [https://portal.office.com/servicestatus misconfigured] a security feature (OCSP stapling) on their servers and is sending you an expired assurance about the validity of their certificate to your firefox. in order to resolve that, please enter '''about:config''' into the firefox address bar (confirm the info message in case it shows up) & search for the preference named '''security.ssl.enable_ocsp_stapling'''. double-click it and change its value to '''false''' - this will degrade a security feature in firefox, so please revisit it in a couple of days again after microsoft has fixed their stuff and turn it back on again then...