Try and disable Firefox 'shield studies'
    
    I've seen a few tests fail lately because Firefox suddenly
    opened a tab with some "Privacy Study" in it and switched to it.
    Per https://bugzilla.mozilla.org/show_bug.cgi?id=1529626 , this
    should be the way to disable these...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>