There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connections and send their own certificate.

SSL_ERROR_NO_CYPHER_OVERLAP Your server apparently doesn't offer any cipher suites necessary to establish a secure https connection that are supported in Firefox. You can check what kind of cipher suites Firefox can make use of by visiting; https://www.ssllabs.com/ssltest/viewMyClient.html

If this is an external device that is connected to your network then you may have to upgrade its security software or firmware. More and more weak cipher suites will be disabled (removed) from current and upcoming Firefox releases and you may not be able to override this. Did this work in previous Firefox versions?

You can install a portable (52 ESR) Firefox version to access websites that do not work with the current Firefox release. The portable version comes with its own profile folder and doesn't interfere with currently installed Firefox versions.

• http://portableapps.com/apps/internet/firefox_portable
• http://portableapps.com/apps/internet/firefox-portable-esr