The ability to play 4K (Ultra High Definition) videos on Firefox depends on your hardware, operating system, and YouTube’s decision to limit the 4K option for certain video codecs.
YouTube offers support two video codecs in different resolutions:
- H264 is available only up to 1080p resolution
- VP9 is available up to 4K resolution
H264 is the more widely available codec, which is supported on lots of machines in hardware, and VP9 requires a faster machine (typically not laptop or dual core machine) because it is still mostly available in software only.
When will I see 4K videos?
Whether or not you see 4K depends on the following:
- Linux: VP9 is enabled in Firefox, thus YouTube shows up to 4K resolution.
- Mac: VP9 is disabled in Firefox, thus only H264 with 1080p will get played.
- Windows: VP9 with 4K depends on the machine (see below).
Firefox's criteria on Windows
Firefox will enable the 4K option for the following scenarios:
- If your computer doesn’t have any hardware decoding capabilities, your computer has to do the decoding in software anyway, so Firefox enables VP9 in this case.
- Your computer doesn't support H264 or H264 was disabled on your machine.
- A small internal benchmark finds your computer is fast enough to support VP9.
In general, and especially on Mac which prefers H264 by default (see above), you can enable VP9 by setting media.mediasource.webm.enabled to true in about:config. But this should only be done by advanced users who understand the impact on battery life and performance.