This might be outdated. NS_ERROR_NET_INADEQUATE_SECURITY indicates that the server initiates a HTTP/2 connection, but Firefox detects an invalid TLS configuration in the server response (server negotiated HTTP/2 with blacklisted cipher suites). This is likely not an issue with the certificate, but this is a problem with the server setup and there are invalid cipher suites for HTTP/2 claimed (INADEQUATE_SECURITY). http://httpwg.org/specs/rfc7540.html#TLSUsage There might also be other software that acts as a MITM and is interfering. When HTTP/2 is enabled and used then the requirements are much stricter than with HTTP/1.1 You can get the NS_ERROR_NET_INADEQUATE_SECURITY error message in case the server isn't configured properly. A workaround to fix this ANNOYING issue is; '''network.http.spdy.enabled.http2 = false''' in about:config Type '''about:config'''<enter> in the address bar. If a warning screen comes up, press the '''I Accept the Risk''' button. At the top of the screen is a search bar.