I've tried the tip in the Firefox help page (delete the 'cert8.db' file), but the problem persists. (I couldn't even get to the Firefox help pages within Firefox, had to use Chrome to get here!) The error message implies it's a problem with the secure web sites themselves, but that can't be right, as I can easily open them with several other browsers. The Firefox help page gives several examples of error messages of the type "sec_error_......", but not the specific one I get: "sec_error_ocsp_old_response". I'm running Windows 8.1, and have tried uninstalling then re-installing Firefox, to no avail. Thanks for any advice!