Managing personal Outlook.com account settings, security, and privacy
It's not a Firefox problem. Lots of chatter in here with people who have no true clue as to the problem. And those of you using Opera and thinking you are still secure, you may not be. In fact, using any browser that gets around this means lower security. Firefox is the secure one on this.
When you try to load the site in another web browser, say Google Chrome or Internet Explorer, it loads fine and without any issues. That doesn't mean Firefox has a problem. The issue is an Invalid OCSP signing certificate owned by Microsoft.
If you check the OCSP range of the certificate, you will notice that it expired on May 28, 2017. While Firefox is strict when it comes to the information, Chrome and IE are not. Google's Chrome browser allows the connection, but considers the connection as insecure instead, while Firefox blocks it outright.
While turning off OCSP Stapling in the Firefox web browser will allow Firefox to connect, it is possible that disabling OCSP Stapling may affect the functionality of other websites that you visit, if they make use of the security feature.
So the reason some other browsers are not getting the error is they are loose with the security standard and allow you to connect to a site with an invalid out-of-date certificate -- that means less secure, not better or more secure. Firefox is strict to the date of the certificate expiring, and blocks the connection.