To protect users from cryptographic attacks that can compromise secure web connections, the popular Firefox browser will block access to HTTPS servers that use weak Diffie-Hellman keys.
Diffie-Hellman is a key exchange protocol that is slowly replacing the widely used RSA key agreement for the TLS (Transport Layer Security) protocol. Unlike RSA, Diffie-Hellman can be used with TLS’s ephemeral modes, which provide forward secrecy — a property that prevents the decryption of previously captured traffic if the key is cracked at a later time.
However, in May 2015 a team of researchers devised a downgrade attack that could compromise the encryption connection between browsers and servers if those servers supported DHE_EXPORT, a version of Diffie-Hellman key exchange imposed on exported cryptographic systems by the U.S. National Security Agency in the 1990s and which limited the key size to 512 bits. In May 2015 around 7 percent of websites on the internet were vulnerable to the attack, which was dubbed LogJam.