This means that your browser could not verify the site's SSL certificate with the issuer's OCSP server. It could be that the cert has been revoked, or the OCSP

This means that your browser could not verify the site's SSL certificate with the issuer's OCSP server. It could be that the cert has been revoked, or the OCSP server is broken, or even that the CA changed the location of the OCSP server (Firefox permenantly caches the server's IP address to prevent certain types of attacks, so this is a pretty bad thing for the CA to do, but it does happen.)