There's a Firefox bug where it hangs for a long time when trying to
connect to Cockpit over TLS.
This is a Firefox issue, and has to do with the TLS certificates that
Cockpit automatically-generates. The same thing happens with Firefox use
of these self-signed certificates on apache or other servers.
Cockpit 0.45 has a work-around to generate certificates slightly
differently so that Firefox doesn't fall over.
To regenerate problematic certificates you can:
$ sudo rm /etc/cockpit/ws-certs.d/~self-signed.cert
$ sudo systemctl restart cockpit