Security News > 2021 > December > Firefox users can't reach Microsoft.com — here's what to do
Specifically, the error code 'MOZILLA PKIX ERROR OCSP RESPONSE FOR CERT MISSING' and the message "The OCSP response does not include a status for the certificate being verified," help trace down the cause of the issue.
The Online Certificate Status Protocol is a way for browsers and other client-side applications to check if an SSL certificate has been revoked, as an alternative to relying on traditional revocation lists.
The certificates presented to client-side apps come appended with this signed time-stamped response that can be trivially verified by the client-side application to ascertain the certificate's status.
If 'OCSP stapling' is enabled on an application, such as a web browser, the application can decide whether terminate the secure connection for certificates deemed invalid, based on the response attached to the certificate.
If Microsoft.com's SSL certificate is otherwise valid, according to Chrome and Safari, why won't Firefox accept it?
Any certificate containing the SHA-256 hashes, as opposed to the older SHA-1, is deemed invalid and causes Firefox to terminate the connection with the website.