Firefox takes time to fire canplaythrough event

Why firefox takes time to emmit canplaythrough event as compared to other browser with same code base and videos and sometimes does not emmit at all.

This is the code

  function loadLeftVideo() {
    video1.load();
    video1.addEventListener("canplaythrough", function () {
      console.log("Video 1 canplaythrough");
      video1Loaded = true;
    });
  }