This works pretty well.

Some caveats like having to use the tv version of the site in the browser which is understandable due to enabling cast options (you could just have it in a separate tab/window out of the way anyway), the other is that the cast name is Xbox 360, I'm assuming its spoofing the header in your browser so YouTube site thinks it is one. Would be nice to be able to change that but probably not possible.

All in all, this is useful for those that have a pc connected to a nice sound system and the tv is off and you can now cast music videos all from your phone. Nice job.

Edit: Just noticed you have to have the tab active or it just pauses. Is it possible to have it continue working while looking at other tabs?

Edit 2: A workaround for the annoying pausing for now is to have the youtube.tv tab in a new window instead and set this in about:config to false: media.block-autoplay-until-in-foreground

I thought this was just a web shortcut to the Youtube TV link, but it's also doing something to make Youtube think this is a TV so it actually works

can cast to 4k no problem but noticed that any video that runs 60 fps defaults to 720p. makes some videos unwatchable because of this issue