I first saw this error last night first with tumblr and then tried to go to the mozilla support site to see what the problem is but got the same error. So then with the help of Google, I discovered going into the "about:config" and setting the "security.ssl.enable_ocsp_stapling" to FALSE to work as a temporary fix for my problem and things began to work fine. Today, I set "security.ssl.enable_ocsp_stapling" back to TRUE but still got the same "Invalid OCSP signing certificate in OCSP response. (Error code: sec_error_ocsp_invalid_signing_cert)" message when opening tumblr and mozilla support. The solution for most people seemed to be to give it time but I wonder if I'm having a different problem to others since sites like twitter and droxbox appeared broken when I tried opening them. But when "security.ssl.enable_ocsp_stapling" was set to FALSE, they seem to work fine.