User Agent: Mozilla/5.0 (Windows NT 5.1; rv:40.0) Gecko/20100101 Firefox/40.0 Build ID: 20150812163655 Steps to reproduce: Youtube iframe embedded HTML5 player buffering in Firefox version 38 through 40. Reproduced on OS XP Pro SP3. Actual results: Immediately after programmatically positioning Youtube playback head and pausing video, Firefox buffers the video at up to 100% CPU without stopping, even after video has already completely loaded. Browser then starts locking up. Expected results: Very low CPU usage during buffering. Chrome and Opera do not reproduce this anomaly. Coudn't test in IE since IE browser not available.

On windows XP you'll be using flash, html5 won't be used

@JVA - Youtube preference for video playback is set to HTML5 player. Debugging features on player are HTML5 player specific. I have not yet encountered the Youtube HTML5 player using the Flash plugin, please validate thoroughly your comment by providing verifiable specs.

When you do a right click on the video, and select "Stats for nerds" what do you see?

@JYA - "Stats for nerds" contains general specs: Video ID: sp0u9S7awxE Dimensions: 687 x 405 Resolution: 0 x 0 Volume: 50% Stream Host: Stream Type: https CPN: 4_Ie1GfMcxo4VqUe Mime Type: video/webm; codecs="vp8.0, vorbis" DASH: no (43) Connection Speed: 0 Kbps Dropped Frames: - "Get debug info" contains data which should be helpful, and in particular: ... "subscribed":null,"lact":4,"live":null,"cl":"102178045","mos":0,"osid":null,"state":"4","vm":"CAE","volume":50,"c":"WEB","cver":"html5","cplayer":"UNIPLAYER","cbr":"Firefox","cbrver":"40.0","cos":"Windows","cosver":"5.1","autoplay":"1","splay":"1","hl":"en_US","cr":"CA","len":"1557.599", ... Here are a few tidbits from dev exchanges dated April+ 2015: ... "Firefox has been working with YouTube on their implementation of this (HTML5) technology, and now that the current version of Firefox implements something called "Media Source Extensions", it is now ready to stream YouTube videos using the native player. That part of it is complete, and it isn't in beta any more: it's fully implemented in current Firefox browsers." Source: https://productforums.google.com/forum/#!topic/youtube/wFsIwaYK8j8 ... "YouTube developers have identified a possible root cause of this problem, and are looking to provide a resolution. At this time, I don't have any information on when it will be available, but will update this thread when we believe the issue is resolved." Source: https://productforums.google.com/forum/#!topic/youtube/wFsIwaYK8j8 ... Could HTML5 video high CPU usage still be a Youtube anomaly? I just recently found Youtube videos that played in sync as two copies side-by-side in the same HTML5 player in Firefox browser. Looks like there is still some work to be done for HTML5 player/Browser compatibility.

Is this reproducible on nightly?

or beta 42 for that matter...