old_contents: (function () {
      // Firefox only TTFI
      // need pref to be activated
      // If the "event" has happend, it will return 0
      const timing = window.performance.timing;
      if (timing.timeToFirstInteractive && timing.timeToFirstInteractive > 0) {
          return Number(
              (timing.timeToFirstInteractive - timing.navigationStart).toFixed(0)
          );
      } else return undefined;
  })();
  
new_contents: (function() {
    // Firefox only TTFI
    // need pref to be activated
    // If the "event" has happend, it will return 0
    const timing = window.performance.timing;
    if (timing.timeToFirstInteractive && timing.timeToFirstInteractive > 0) {
      const ttfi = Number(
        (timing.timeToFirstInteractive - timing.navigationStart).toFixed(0)
      );
      // We have seen cases when TTFI is - 46 years.
      if (ttfi < 0) {
        return 0;
      } else {
        return ttfi;
      }
    } else return undefined;
  })();
  
subject: Fix so we don't report negative values #879 (#880)