I noticed, probably after the recent browser update, most videos will lag in full screen mode. The problem does not occur in Chrome, and when playing a video without full-screen. For example. I'm watching a lecture and the teacher is going back and forth. During that he will freeze and lag, the audio works fine, than it continues. I also noticed that the effect can easily be caused by rewinding back for a few seconds. Not sure if this is a my laptop, youtub's or the firefox problem.