anchor.diff:    name = "${pname}-${version}";
   
     src = fetchurl {
  -    url = "http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/${version}/source/firefox-${version}.source.tar.bz2";
  +    url =
  +      let ext = if lib.versionAtLeast version "41.0" then "xz" else "bz2";
  +      in "http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/${version}/source/firefox-${version}.source.tar.${ext}";
       inherit sha1;
     };
   
  
positive.code:       "--disable-installer"
        "--disable-updater"
        "--enable-jemalloc"
      ]
      ++ lib.optional enableGTK3 "--enable-default-toolkit=cairo-gtk3"
      ++ (if debugBuild then [ "--enable-debug" "--enable-profiling" ]
  
negative.code: 
    firefox = common {
      pname = "firefox";
      version = "41.0";
      sha1 = "0ce7a5ccdf671a6c98eaac07d06d49a895a99449";
    };
  
    firefox-esr = common {
  