I ran the SSLLabs test using the domain name you provided by PM, and Firefox is correct–your certificate is revoked. That domain is using a Let’s Encrypt cert issued on 28 Feb, but at some point after that time it was revoked. The Let’s Encrypt staff on their forums may be able to track down when and why that happened, but the information isn’t public. The most-likely reason for revocation is simply that you requested it. If you didn’t, the only other likely explanation would be that someone else convinced the Let’s Encrypt staff that it shouldn’t have been issued in the first place.
In order to correct this, you’ll need to issue a new certificate.