This article explains why you may see a Secure Connection Failed or a Did Not Connect: Potential Security Issue error page and what you can do.
When a website that requires a secure (https) connection tries to secure communication with your computer, Firefox cross-checks this attempt to make sure that the website certificate and the connection method are actually secure. If Firefox cannot establish a secure connection, it will display an error page.
A Secure Connection Failed error page will include a description of the error, an option to report the error to Mozilla and a button. There is no option to add a security exception to bypass this type of error.
The error page will also include the following information:
Certain secure connection failures will result in a Did Not Connect: Potential Security Issue error page.
The error page will include a description of the potential security threat, an option to report the error to Mozilla and an button to view the error code and other technical details. There is no option to add a security exception to visit the website.
Some websites try using outdated (no longer secure) Transport Layer Security(TLS) mechanisms in an attempt to secure your connection. Firefox protects you by preventing navigation to such sites if there is a problem in securely establishing a connection. Contact the owners of the website and ask them to update their TLS version to a version that is still current and still secure.
Starting in Firefox version 74, the minimum TLS version allowed by default is TLS 1.2. Websites that don't support TLS version 1.2 or higher will display a Secure Connection Failed error page with Error code: SSL_ERROR_UNSUPPORTED_VERSION and a message that This website might not support the TLS 1.2 protocol, which is the minimum version supported by Firefox. The error page may also include a button, that will allow you to override the minimum TLS requirement; however, Mozilla plans to remove this option and permanently disable TLS 1.0 and 1.1 in a future version of Firefox.
Other websites may require HTTP Strict Transport Security (HSTS) and will not allow access with an insecure connection.
Many security products use a feature that intercepts secure connections by default. This can produce connection errors or warnings on secure websites. If you see secure connection errors on multiple secure websites, updating your security product or modifying its settings may resolve the issue.
Alternatively, you can uninstall third-party security software and use Windows Defender, the built-in antivirus on Windows 8 and Windows 10.
Firefox uses certificates on secure websites to ensure that your information is being sent to the intended recipient and can't be read by eavesdroppers. An incorrect system date can cause Firefox to detect that the website's security certificate is expired or invalid. Make sure your computer is set to the correct date, time and time zone.