Start Firefox as soon as X starts

There's no reason to wait for an incoming Marionette connection to start
Firefox.  Anyway, starting it earlier avoids a bunch of "connection
refused" spam from `systemd-socket-proxyd`.

overlay/home/user/.xinitrc

@@ -3,5 +3,5 @@
 xset s off
 xset -dpms
 systemctl --user import-environment DISPLAY
-systemctl --user start firefox-marionette.socket
+systemctl --user start firefox-marionette.socket firefox
 exec openbox