anchor.diff:  
   let
   
  -common = { pname, version, sha1 }: stdenv.mkDerivation rec {
  +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 sha1;
  +    inherit sha256;
     };
   
     buildInputs =
  
positive.code: 
    firefox = common {
      pname = "firefox";
      version = "41.0.2";
      sha1 = "5e8243cbbd3ea306bd1e5f1b16079bdcc9af95a4";
    };
  
    firefox-esr = common {
      pname = "firefox-esr";
      version = "38.3.0esr";
      sha1 = "57d2c255348ac13b6ffbb952c5e0d57757aa0290";
    };
  
  }
  
negative.code:     ''
        mkdir ../objdir
        cd ../objdir
        if [ -e ../${name} ]; then
          configureScript=../${name}/configure
        else
          configureScript=../mozilla-*/configure
        fi
      '';
  
    preInstall =
  