История изменений

Там не только это же в гайде:

You need to create a new preference to do so:
Type about:config and hit enter.
Confirm you will be careful.
Right-click and select New > Boolean.
Name the preference media.mediasource.ignore_codecs.
Set its value to True.

Update: Linux userrs may change the following preferences as well:
media.mediasource.mp4.enabled to true
media.fragmented-mp4.* to true
media.fragmented-mp4.use-blank-decoder to false

If you go back to YouTube's HTML page, you should see all six technologies listed as supported (in green).

Там не только это же в гайде:

You need to create a new preference to do so:

Type about:config and hit enter. Confirm you will be careful. Right-click and select New > Boolean. Name the preference media.mediasource.ignore_codecs. Set its value to True.

Update: Linux userrs may change the following preferences as well:

media.mediasource.mp4.enabled to true media.fragmented-mp4.* to true media.fragmented-mp4.use-blank-decoder to false

If you go back to YouTube's HTML page, you should see all six technologies listed as supported (in green).