Ever since the HTML5 player has taken over Youtube, I've had problems with it perma-freezing my computer as soon as any window was opened. Until recently I got around that by setting media.windows-media-foundation.enabled to false, forcing Youtube to use the Flash player. Lately, though, it's somehow gone back to the HTML5 player with 360p resolution only, so I went digging through the config file again in search of solutions. That's when I found two settings I hadn't noticed before, media.hardware-video-decoding.enabled (which I found true) and media.hardware-video-decoding.failed (which I also found true, though I hadn't touched it before). Re-enabling windows-media-foundation caused the hard-freezes to reoccur, but I discovered that by setting media.hardware-video-decoding.enabled to false I was able to leave the windows-media-foundation option true, and I can now use Youtube's HTML5 player in all resolutions. I conclude from this that the media.hardware-video-decoding.failed setting is bugged somehow, since it didn't seem to prevent the freezes.