The ciphers supported by Firefox are
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 (0xc02b) TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (0xc02f) TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 (0xcca9) TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 (0xcca8) TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 (0xc02c) TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (0xc030) TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA (0xc00a) TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA (0xc009) TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (0xc013) TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (0xc014) TLS_DHE_RSA_WITH_AES_128_CBC_SHA (0x33) TLS_DHE_RSA_WITH_AES_256_CBC_SHA (0x39) TLS_RSA_WITH_AES_128_CBC_SHA (0x2f) TLS_RSA_WITH_AES_256_CBC_SHA (0x35) TLS_RSA_WITH_3DES_EDE_CBC_SHA (0xa)
The last three ones are reported as weak by https://www.ssllabs.com/ssltest/viewMyClient.html
The author should add at least one of the others to the web site SSL configuration.