Has anyone else experienced a degraded symmetric key exchange when using FF-31
vice FF24?

When I use FF24 then I get a symmetric type of AES-256 (Very Strong) rating in Calomel 0.62. When I switch to FF31 and connect to exactly the same server host and url then in Calomel 0.62 I see this instead:
TLS_DHE_RSA_WITH_AES_256_CBC_SHA (Very Weak).

I am not altering any of the configuration options in FF between version trials.