Hi. When i try to play videos on Youtube or Vimeo, they won't play if they are 2K (1440p) or 4K (2160p), AND the codec used is AVC/MP4. The video will play fine on resolutions up to and including 1080p. Selecting a higher resolution, or viewing fullscreen and having it auto-select 2K or 4K, and the video immediately stops playing and never resumes. Selecting 1080p again will continue the video playback. If the codec is WebM/VP9 however, it plays just fine even at 2K or 4K. For Youtube I can see this is the case by checking the 'stats for nerds' info. For Vimeo this is a guess* but it shows the problem is broader than just Youtube. For instance, this video won't play at the 2K-setting: https://vimeo.com/163590531 https://www.youtube.com/html5 shows me 7 blue boxes with checkmarks. So everything is supported. These settings are all enabled in config: media.mediasource.mp4.enabled;true media.mediasource.webm.enabled;true media.mediasource.enabled;true .*Disabling mediasource.mp4 removes resolutions above 1080p from the available options. Disabling hardware acceleration in the browser does not fix the problem. I'm using the latest Firefox, and recent Nvidia drivers. 150 Mbit connection. I would like to be able to view the 2K and 4K AVC/MP4 videos. Especially on Youtube where new videos appear in this format. If the video becomes popular Youtube will replace it with a WebM/VP9 version, but this can take days, if it happens at all. Meanwhile I'm stuck at 1080p, and having to manually downgrade the setting before I can watch, because Youtube auto-selects 1440p for my machine (2560 x 1600 display). And I want to watch it at 1440p. What can I do to remedy this that I haven't tried already? Thanks. Edit 1: Just tried watching the vimeo video linked above in a VM with Windows 10 and Edge, and it plays just fine at 2K there. I also tried a Windows 7 VM with Firefox, and it won't play at 2K. And I tried a Windows XP VM with an old IE, and even that plays at 2K. This strengthens my belief it is somehow a Firefox issue. Edit 2: OK. Now I'm confused. I tried Win 10 + Firefox = works. Win 10 + Chrome = works. Win 7 + Chrome = works. Win 7 + Firefox = doesn't work! Win 7 + Firefox and all add-ons disabled = also doesn't work. All this in VM. So Win 10 + Firefox in VM works at 2K. Other browsers in Win 10, and Win 7 work at 2K. But Win 7 + Firefox doesn't work at 2K in VM and on my host machine. The VM probably excludes my graphics drivers as the cause. Is the problem a combination of Windows 7 + Firefox??