Aliasing with overlays

configuration.nix

{ config, pkgs, ... }:

let channels = rec {
  pkgs-unstable = import (fetchTarball https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz) { config.allowUnfree = true; };
  pkgs-master = import (fetchTarball https://github.com/NixOs/nixpkgs/archive/master.tar.gz) {
    config.allowUnfree = true;
    overlays = [
      (self: super: {
        firefox-master = super.firefox.override {
          browserName = "firefox";
          desktopName = "firefox-master";
        };
      })
    ];
  };
};
in with channels;

...

environment.systemPackages = with pkgs; [
...
    firefox-esr
    pkgs-master.firefox-master
...

# Tried various variations (e.g. just `super.firefo`, overriding browserName, ...) but it's still a collision and I can't get it to get /run/current-system/sw/bin/...
collision between `/nix/store/3b3wp27mx4xqwyav3c8vn1675y539l4m-firefox-57.0.1/bin/firefox' and `/nix/store/0ncpkak56c3p8ac6aaa3pca6hm8xyjf5-firefox-52.0.2esr/bin/firefox'