code: function(opt_exe) {
    /** @private {(string|undefined)} */
    this.exe_ = opt_exe;
  
    /** @private {!Array.<string>} */
    this.args_ = [];
  
    /** @private {!Object.<string, string>} */
    this.env_ = {};
    Object.keys(process.env).forEach(function(key) {
      this.env_[key] = process.env[key];
    }.bind(this));
    this.env_['MOZ_CRASHREPORTER_DISABLE'] = '1';
    this.env_['MOZ_NO_REMOTE'] = '1';
    this.env_['NO_EM_RESTART'] = '1';
  
    /** @private {promise.Promise.<!exec.Command>} */
    this.command_ = null;
  }
docs: Manages a Firefox subprocess configured for use with WebDriver.
  @param {string=} opt_exe Path to the Firefox binary to use. If not
  specified, will attempt to locate Firefox on the current system.
  @constructor