My RX 580 arrived yesterday and replaced my old GTX 770. It does a fantastic job at gaming so far and overall I'm satisfied with it. My monitor is a 1440p@144Hz monitor that supports FreeSync, which is the primary reason why I upgraded to an AMD card instead of a new nVidia one. But there's one huge issue that doesn't make any dang sense to me: Watching YouTube videos at 720p or above in full screen puts severe stutter to them. But that's not the main head scratcher: it only happens when Firefox is the active window! So when I turn on full screen and then click on a blank space on my secondary monitor, the video playbacks completely fine. When I click back into the video the stuttering kicks in again. It's not like the card isn't capable of rendering the videos fast enough. Heck, it does 4K@60FPS just fine when I click away from video. It's only when I set focus to Firefox that it does this obnoxious stuff. The problem definitely lies within Hardware Acceleration (not a real surprise, really). It does not happen in Safe Mode, but manually disabling all add-ons and themes changes nothing. Disabling HA fixes the issue somewhat, but it introduces consistent sluggishness to Firefox as a whole, so not a suitable workaround. It also doesn't happen in Chrome or even Edge. I've already updated my drivers to the latest version and also played around with FreeSync, GPU scaling, and whatnot. All to no effects. Any ideas? DxDiag file: https://pastebin.com/FyeuZaWn