Enable 4k UHD video streaming in Firefox

By default, Firefox does not support the VP9 WebM codec sites like YouTube use to deliver 4k quality video. If you visit a video that supports 4k you won't even see the option available under the Quality menu.

Test video: https://www.youtube.com/watch?v=Bey4XXJAqS8

To enable it:

1. visit about:config
2. search for webm
3. double click media.mediasource.webm.enabled

And now refresh YouTube

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