firefox.nix

{ pkgs ? import nixpkgs {}
, nixpkgs ? builtins.fetchTarball {
    url = "https://releases.nixos.org/nixpkgs/nixpkgs-20.03pre206743.af57b17404e/nixexprs.tar.xz";
    sha256 = "0zksmczv3lyaldr4m7p1ziam0hxiafbngamli9fmzzjql2vvy0ra";
  }
}:

with pkgs;

stdenv.mkDerivation rec {
  name = "firefox-${version}";
  version = "71.0";
  buildInputs = [ undmg unzip ];
  sourceRoot = ".";
  phases = [ "unpackPhase" "installPhase" ];
  installPhase = ''
    mkdir -p "$out/Applications"
    cp -r Firefox.app "$out/Applications/firefox.app"
    mkdir -p "$out/bin"
    cp -r "$out/Applications/firefox.app/Contents/MacOS/firefox" \
        "$out/bin/firefox"
  '';

  src = fetchurl {
    name = "firefox-${version}.dmg";
    url = "https://download-installer.cdn.mozilla.net/pub/firefox/releases/${version}/mac/en-GB/Firefox%20${version}.dmg";
    sha256 = "1kknqpsymh9jyvabhvg47bjgbazji7kpacxdvp9pb97gsv73fym8";
  };

  meta = with stdenv.lib; {
    description = "The Firefox web browser";
    homepage = https://www.mozilla.org/en-GB/firefox;
    maintainers = with maintainers; [ cmacrae ];
    platforms = platforms.darwin;
  };
}

gistfile1.txt

$ nix-build firefox.nix

$ ls -l /nix/store/12p7g0a28rg3prs7xdvv9qpnn0mgkki7-firefox-71.0/Applications/firefox.app/Contents/MacOS/firefox
-r-xr-xr-x 1 root wheel 36688 Jan  1  1970 /nix/store/12p7g0a28rg3prs7xdvv9qpnn0mgkki7-firefox-71.0/Applications/firefox.app/Contents/MacOS/firefox