Watching YouTube videos in 1080p60 on Firefox

[colonel_mortis]

Warning: This method enables experimental code that is slightly buggy (eg when changing position in the video). Use at your own risk. I have added steps to revert to the original behaviour at the bottom.

Supported browsers: Firefox 35+ (maybe 34 as well). I strongly advise that if you follow these instructions, you are using a Nightly, Aurora or Developer Firefox build to ensure that bugs are fixed in a timely manner.
NOTE: Within a few release cycles, the necessary code will be enabled by default, and you will not need to follow these steps to enable 1080p60fps video in Firefox. This guide shows you how to enable the experimental features necessary.
 

 
What this guide does: This guide will walk you through enabling an experimental feature called "Media Source Extensions". It basically allows YouTube to split the video up into sections and only send you the sections that you need to play the next bit of the video. It is also known as DASH playback, and YouTube requires it to be enabled to use resolutions other than 480p and 720p and frame rates greater than 30fps.
 
To follow this guide, you will need to open a new tab and go to about:config (type it into the address bar), read and accept the warning, then in the search box, search for media.mediasource
You should see at least one option listed on that page. Leave it open and follow the guide. Don't change anything on that page unless you know what they will do because otherwise bad things will happen.

• Check how much your browser already supports it - go to http://www.youtube.com/html5.
  This is what it looks like in the current Nightly for me before following this guide:
  
  If anything on the top row has an !, there is something wrong with your FF installation (it may just be out of date), and you need to resolve that before you can follow this guide.
  If everything on that page is supported, there is nothing left for you to do.
• If "Media Source Extensions" or "MSE & WebM VP9" are flagged as unsupported, in about:config you need to double click on the entry that says media.mediasource.enabled. It should change the "value" column to "true".
• If "MSE & H.264" is unsupported, you will need to either set media.mediasource.mp4.enabled to true (double click) or media.mediasource.ignore_codecs to true. If neither of these show up on the page, you will need to create them - right click on the page and select "New" -> "Boolean". Name it one of the above, press OK, then set the value to true.
• Go back to http://www.youtube.com/html5 and check that everything is marked as compatible, like it is for me:
  
  and make sure that you are set to use the HTML5 player (the default player will not work).

If you have any issues, I'll try to answer them. This guide is based off my own experiences, and is a result of a fair bit of research. Note that I am using Aurora/Dev Edition for my primary browser, and that is where I researched for this guide, but some of the screenshots are from nightly, and it should work on nightly.
Please share whether it worked for you or not in this thread (including your FF version) so that other people can troubleshoot.
 
To revert back to the original behaviour, go back to about:config and set media.mediasource.enabled to false, or tell YouTube to use the default player (note: this may not always be possible).

[Sharif]

Nice

Thanks for the guide 

[rambi36]

Thanks dude! Very useful!

[Guest]

I loved Firefox, but the new Safari is SOOOOO efficient in rendering.

[Aniallation]

 

How did you get dark themed YouTube?

[colonel_mortis]

How did you get dark themed YouTube?

I was wondering when this would appear

I used this awesome user style, although I have tweaked it a bit myself to make it work better with recent youtube changes.

[NeatSquidYT]

Thanks My "MSE and WebM VP9" wasn't enabled, but still worked

[swifty7]

OMG!!  OMG!! what the hell is wrong with FF by not having it enabled in first place.  THANK YOU!!  it made a world of difference. The 60FPS videos are breathtaking.   Now!  If only I could make VLC player to stream youtube @ 60fps.

 

 

thanks again OP

[colonel_mortis]

OMG!!  OMG!! what the hell is wrong with FF by not having it enabled in first place.  THANK YOU!!  it made a world of difference. The 60FPS videos are breathtaking.   Now!  If only I could make VLC player to stream youtube @ 60fps.

 

 

thanks again OP

The features will be enabled by default soon (I think they are in the current alpha builds), but at the moment they're still experimental so they are disabled by default on the release channel.