An error occurs in the elliptic curve point addition algorithm that uses mixed Jacobian-affine coordinates where it can yield a result "POINT_AT_INFINITY" when it should not. A man-in-the-middle attacker could use this to interfere with a connection, resulting in an attacked party computing an incorrect shared secret. This vulnerability affects Firefox < 55.
https://www.mozilla.org/security/advisories/mfsa2017-18/
https://bugzilla.mozilla.org/show_bug.cgi?id=1352039