With well-run websites, the problem isn't the website itself, it's nearly always that an intermediary -- which could be your security software, a proxy server, malware on your system, or a hijacked connection -- is presenting Firefox a fake certificate for the site so that it can intercept and decrypt your connection. Could you take a moment to let us help you determine the source of the error. Often a certificate error page will have an "Advanced" button you can click to obtain more technical details about the problem. This article addresses one of the most common codes you might find: [[How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites]] Does that seem to apply to your Firefox? If you are getting a different code, could you copy/paste the full explanation into a reply here?