Ah, you're using the Go TLS package? That makes sense ;-).
As it turns out the latest version of Firefox still connects to servers which don't indicate secure renegotiation; the parent poster caused the problem by mucking around with the TLS settings in about:config.
(The TLS stack doesn't support renegotiation at all, so it's not vulnerable, but a client can't know that unless it echos the extension in question.)