TBB 3.0beta1 (and I'm guessing all ESR-based TBBs) do not support TLS 1.2
As confirmed via https://www.ssllabs.com/ssltest/viewMyClient.html
Recent non-ESR versions of Firefox can be configured to support TLS 1.2 by changing the security.tls.version.max preference to 3.
More details on the Firefox experience can be found at https://bugzilla.mozilla.org/show_bug.cgi?id=861266