gistfile1.nix

{ pkgs }: {

  packageOverrides = pkgs: rec {
    ryantmEnv = pkgs.buildEnv {
      name = "ryantm";
      paths = with pkgs; [
        firefox
        chromium
        thunderbird
      ];
    };
  };
}

gistfile2.txt

building path(s) `/nix/store/0w2r7f8hpj71ya2lfzlb7ssmwlargjml-ryantm'
building /nix/store/0w2r7f8hpj71ya2lfzlb7ssmwlargjml-ryantm
created 11 symlinks in user environment
building path(s) `/nix/store/qy6aw1yczxp823y6ssj899k2qmvjw894-user-environment'
collision between `/nix/store/7yrxr3yd4c3vgx48c89xi20hybcmdy6n-firefox-31.0/bin/firefox' and `/nix/store/0w2r7f8hpj71ya2lfzlb7ssmwlargjml-ryantm/bin/firefox'; use `nix-env --set-flag priority NUMBER PKGNAME' to change the priority of one of the conflicting packages
builder for `/nix/store/jcnmyvdxpmplarlacs2s6iji9d3w79ln-user-environment.drv' failed with exit code 255
error: build of `/nix/store/jcnmyvdxpmplarlacs2s6iji9d3w79ln-user-environment.drv' failed