nightwatch.json

{
  "src_folders" : ["tests"],

  "webdriver" : {
    "start_process": true,
    "server_path": "node_modules/.bin/geckodriver",
    "port": 4444,
    "cli_args": [
      "--port=4444"
    ]
  },

  "test_settings" : {
    "default" : {
      "desiredCapabilities": {
        "browserName": "firefox"
      }
    }
  }
}