See https://bugzilla.mozilla.org/show_bug.cgi?id=1735738 for reference.
_getQueryStringParameters: function Driver_getQueryStringParameters() {
      var queryString = window.location.search.substring(1);
      var values = queryString.split("&");
      var parameters = {};
      for (var i = 0, ii = values.length; i < ii; i++) {
        var value = values[i].split("=");
        parameters[unescape(value[0])] = unescape(value[1]);
      }
      return parameters;
    },
    run: function Driver_run() {
      var self = this;
      window.onerror = function (message, source, line, column, error) {
        self._info(
          "Error: " +
            message +
            " Script: " +
            source +
            " Line: " +
            line +
            " Column: " +
            column +
            " StackTrace: " +
            error
        );
      };
      this._info("User agent: " + navigator.userAgent);
      this._log(`Harness thinks this browser is ${this.browser}\n`);
      this._log('Fetching manifest "' + this.manifestFile + '"... ');
      var r = new XMLHttpRequest();