That is FF warning of an issue with the sites certificate. Whether it is accurate or not I can't say. Can you provide an address so we can check it with our FF 18.01?
Have a look here, it is for an older version, but is still valid certificate error | Firefox Support Forum | Mozilla Support
Not the caution, if you get warned of a problem with a certificate, and choose to ignore it, you should be certain it is safe (and how can you be?)