User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 Steps to reproduce: System Specs: OS : Windows 10 Pro 64bit (OS Build 15063.463) CPU : i7 3770S Mem : 8Gb GPU : nVidia 1030 GT (GeForce driver Aug-2017 - version 22.21.13.8528) Firefox: 55.0.3 64Bit Eng-US 1. Clean install of latest Firefox 55.0.3 64bit Eng-US (Prior to this install Mozilla's folders in appdata were deleted and no add-on's installed) 2. Open (as example) this video at that point of time the video jumps to: https://www.youtube.com/watch?v=LeiFF0gvqcc&t=175 3. Enter full screen mode. 4. Click progress bad to jump back to the sand part few times. 5. Video begins to stutter. 6. Aside from stutters, overall HTML5 videos playback is not smooth, 60fps videos perform close to 30fps Actual results: 1. Before this fresh setup, attempted to force Flash, instead of HTML5 on YouTube. Forcing Flash solved the frame drops and solved the stutters. However, the add-on that forces Flash is not perfect, plus, why wouldn't Firefox run HTML5 playback smoothly ? 2. I never used Chrome, i'm a big fan of Mozilla and Firefox, but i had to isolate the problem further, installed Chrome 60, and was surprised, HTML5 videos played on Chrome, run smooth as intended, no stutters. 3. Firefox HTML5 full screen playback on YouTube, can be described as follows: Video randomly jumps 3 frames forward and then 2 to 3 frames back, very fast, and quite annoying. As if Firefox HTML5 playback is messed up, where frames do not progress at the right order and speed, and sometimes frames repeat back and forth. Expected results: 1. Smooth HTML5 playback on YouTube and Twitch in full screen. 2. Perform same or better than Chrome 60 while playing HTML5 videos on YouTube or Twitch 3. Firefox should not have HTML5 stutters or frame drops at full screen.

May be related to bug 1397307.

Per [1], this should be the same issue. [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1383292#c18