anchor.diff:  
     firefox = common {
       pname = "firefox";
  -    version = "41.0.2";
  -    sha1 = "5e8243cbbd3ea306bd1e5f1b16079bdcc9af95a4";
  +    version = "42.0";
  +    sha256 = "1bm37p1ydxvnflh7kb52g6wfblxqc0kbgjn09sv7g0i9k5k38jlr";
     };
   
     firefox-esr = common {
       pname = "firefox-esr";
  -    version = "38.3.0esr";
  -    sha1 = "57d2c255348ac13b6ffbb952c5e0d57757aa0290";
  +    version = "38.4.0esr";
  +    sha256 = "1izj0zi4dhp3957ya1nlh0mp6gyb7gvmwnlfv6q1cc3bw5y1z2h2";
     };
   
   }
  
positive.code:     ''
        mkdir ../objdir
        cd ../objdir
        configureScript=../mozilla-*/configure
      '';
  
    preInstall =
  
negative.code: 
  let
  
  common = { pname, version, sha256 }: stdenv.mkDerivation rec {
    name = "${pname}-${version}";
  
    src = fetchurl {
      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 sha256;
    };
  
    buildInputs =
  