The remote host has a web browser installed that may accept MD5 signatures within TLS 1.2.
The version of Firefox is prior to 43.0.2 and is affected by a collision-based forgery vulnerability, known as SLOTH (Security Losses from Obsolete and Truncated Transcript Hashes), in the TLS protocol due to accepting RSA-MD5 signatures in the server signature within the TLS 1.2 ServerKeyExchange messages during a TLS handshake. A man-in-the-middle attacker can exploit this, via a transcript collision attack, to impersonate a TLS server. (CVE-2015-7575)
Upgrade to Firefox 43.0.2 or later.