On both Kali and Kubuntu, I get this in the log:
647896709734 geckodriver INFO Listening on
127.0.0.1:33721647896709961 mozrunner::runner INFO Running command: "/usr/bin/firefox" "--marionette" "-no-remote" "-profile" "/tmp/rust_mozprofiledgOEk7"
Running Firefox as root in a regular user's session is not supported. ($XAUTHORITY is /home/joseph/.Xauthority which is owned by joseph.)