There seems to be something wrong on the Outlook servers. Hopefully they fix this quickly on the server. I've see a few reports similar to yours and I can confirm it myself.

This looks like a problem with OCSP stapling on the server because it works when I disable this feature in Firefox.

• https://en.wikipedia.org/wiki/OCSP_Stapling
• https://blog.mozilla.org/security/2013/07/29/ocsp-stapling-in-firefox/

You can temporarily toggle this pref to false on the about:config page to see if disabling OCSP Stapling works for you. It is best to reset this pref via the right-click context menu to true once you are done with the this website.

• security.ssl.enable_ocsp_stapling = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

Thanks for the response, glad (in a way, although it's very inconvenient) that it's not just me. However, I can open Hotmail on another browser- Epic and on my phone. Strange that only on Firefox is it a problem?

Firefox seems to check the OCSP revoked status on every access and in this case the server is sending a cached OCSP Stapling response that is signed with an expired certificate.

• https://portal.office.com/servicestatus

Firefox seems to be more persistent and refuses to access the website. Other browser may be more forgiving or aren't using this feature in case of older browsers.