Run bare Firefox on Weston (not X.org) on F37+
    
    This should hopefully avoid RHBZ#2119970 , plus it's just a more
    modern way of doing it which is more in line with how the rest
    of the distro works and less likely to be broken due to being
    a pretty obscure thing to do. We can't do it this way on F35/F36
    because their versions of weston do not have autolaunch support.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>