Check the date and time and time zone in the clock on your computer: (double) click the clock icon on the Windows Taskbar. Check out why the site is untrusted and click "Technical Details to expand this section.<br>If the certificate is not trusted because no issuer chain was provided (sec_error_unknown_issuer) then see if you can install this intermediate certificate from another source. You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates. *Click the link at the bottom of the error page: "I Understand the Risks" Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate". *Click the "View..." button and inspect the certificate and check who is the issuer of the certificate. You can see more Details like intermediate certificates that are used in the Details pane. If <b>"I Understand the Risks"</b> is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and use "This Frame: Open Frame in New Tab". *Note that some firewalls monitor (secure) connections and that programs like Sendori or FiddlerRoot can intercept connections and send their own certificate instead of the website's certificate. *Note that it is not recommended to add a permanent exception in cases like this, so only use it to inspect the certificate. ---- You can try to rename the cert8.db file (cert8.db.old) and delete the cert_override.txt file in the Firefox profile folder to remove intermediate certificates and exceptions that Firefox has stored. If that has helped to solve the problem then you can remove the renamed cert8.db.old file. Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previously stored intermediate certificates. Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.