Do any of the web admins here know about "Hellenic Academic and Research Institutions CA"? A fair amount of the universities in Central and Eastern Europe that I see pop up warnings in LibreWolf because their security certificates are suspicious. When I view the cert, it says it's from "Hellenic Academic and Research Institutions CA".
So I don't know if the sites are truly compromised or if LibreWolf just doesn't like certs signed by this org.