I am not sure about it, but my guess is that it would use the JRE's default version, which is TLS 1.2 for Java 8 (and I think TLS 1.1 for Java 7, but not sure).
See also
JDK 8 will use TLS 1.2 as default (Java Platform Group, Product Management blog)
At least I don't know any further Openfire configuration for a TLS version, so assume it's using the default.