You would indeed, which is part of the reason I released this service + libraries, so some enterprising developer can build a nice alerting service with it for folks just like you!

> Do I tell the mozilla that "someone issued cert id 4d8effdd25 for my nextcloud installation

Not exactly, I believe you'd probably contact the certificate issuer who issued the original certificate to have them issue a revocation, but my sincere hope is that folks running CAs will eventually come up with some better method for flagging certificates as bad/malicious than "just email Symantec support", since I wouldn't wish that on anyone.