configuration.nix

conf.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; };
};
in with channels;

...

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

# Tried searching for the file, could find it:
$ sudo find /run/current-system/sw/bin -lname /nix/store/0ncpkak56c3p8ac6aaa3pca6hm8xyjf5-firefox-52.0.2esr/bin/firefox
/run/current-system/sw/bin/firefox
$

$ sudo find /run/current-system/sw/bin -lname /nix/store/y43dyj2b91ydb2dn2mr8cm8zgjxmmswi-firefox-57.0/bin/firefox 
$