In response to recent developments attacking Diffie-Hellman key exchange (https://weakdh.org/) and to protect the privacy of Firefox users, we have increased the minimum key size for TLS handshakes using Diffie-Hellman key exchange to 1023 bits. A small number of servers are not configured to use strong enough keys. If a user attempts to connect to such a server, they will encounter the error “ssl_error_weak_server_ephemeral_dh_key”.

https://blog.mozilla.org/security/2016/09/30/mitigating-logjam-enforcing-stronger-diffie-hellman-key-exchange/