Kev,

It does say that there's "experimental" support in V. 52, so
you might be able to turn it on in about:config somewhere.

Googleing showed me that setting "security.tls.version.max" to 4 should
enable TLS 1.3 (if present ofcourse). Alas, there seems to be no way to
actually check if it works ... :-(

Regards,
Rudy Wieser