This appears to be a problem with certain Symantec certificates. The web server tells Firefox that it supports OCSP stapling, but the verification returned with the certificate is invalid. When I check sites that had this problem reported on [https://www.ssllabs.com/ssltest/], in nearly every case in the last table in the report I found: OCSP stapling: Invalid - Failed to validate response I'm not a webserver admin, so I don't know whether this might be an out-of-date file or a glitch at Symantec or something else. I can say that if OCSP stapling is disabled in Firefox, and presumably if it is disabled on the webserver, the problem goes away.