Tweak the Firefox config override stuff again based on research
    
    Wow, so the real way to do config overrides is, uh...obscure.
    One file pointing to another file, both with mandatory comment
    lines and one with a weird required value. Wat. Anyway, this
    works in a VM. I still don't know why the policy for the first
    run page isn't working as advertised.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>