anchor.diff:        "--with-system-libvpx"
         # "--with-system-png" # <-- "--with-system-png won't work because the system's libpng doesn't have APNG support"
         "--enable-startup-notification"
  -      "--enable-system-cairo"
         "--enable-system-ffi"
         "--enable-system-hunspell"
         "--enable-system-pixman"
         "--enable-system-sqlite"
  -      "--with-system-libvpx"
         "--disable-crashreporter"
         "--disable-tests"
         "--disable-necko-wifi" # maybe we want to enable this at some point
         "--disable-installer"
         "--disable-updater"
  -    ];
  +    ] ++ optional useSystemCairo "--enable-system-cairo";
   
   
     xulrunner = stdenv.mkDerivation rec {
  
positive.code: }:
  
  assert stdenv.gcc ? libc && stdenv.gcc.libc != null;
  
  rec {
  
    firefoxVersion = "21.0";
  
  
negative.code: 
      enableParallelBuilding = true;
  
      patches = optional useSystemCairo ./system-cairo.patch; # probably in 22
  
      preConfigure =
        ''
  