FirstPaintTime for Firefox

moz_first_paint.js

var firstPaint = 0, firstPaintTime, paint;
function paintHandler() {
  paint = window.performance.now()
  if (firstPaint === 0) {
    firstPaint = paint;
    firstPaintTime = firstPaint + window.performance.timing.navigationStart;
    window.performance.timing.firstPaint = firstPaint;
    window.performance.timing.firstPaintTime = firstPaintTime;
  }
};
window.addEventListener("MozAfterPaint", paintHandler, false);