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If a website requires a secure connection that cannot be established, Firefox will not connect and will show you an error page. Learn more.
Learn about the error codes SEC_ERROR_UNKNOWN_ISSUER MOZILLA_PKIX_ERROR_MITM_DETECTED and ERROR_SELF_SIGNED_CERT on HTTPS websites and how to troubleshoot.
Learn more about common misconceptions about Private Browsing in Firefox.
Firefox issues an insecure password warning when a web page's login form is not secure and your information could be stolen.