Use rpm-ostree usroverlay for Firefox config on rpm-ostree
    
    Editing the files under /ostree/deploy doesn't work on Rawhide
    any more because it's been made read-only. Thanks to Timothée
    Ravier for pointing out this way of doing it, which is better
    anyway and avoids awful quotation issues in the code.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>