Google is currently experimenting with a slick new transparent player. If you want to activate it for yourself, you can do so with a simple cookie edit.
The new transparent player is mostly an aesthetic upgrade to YouTube (though after years with the same player, it’s more than welcome), however it comes with one feature that most other players have had for a while: it disappears while you’re watching a video and only re-appears when you hover over the window. If you want to try it out, you just need to edit a single cookie.
A cookie that YouTube sets determines whether you get the new player interface or the old. Below you find instructions on how to modify the cookie to set the new player interface on YouTube in Firefox and Chrome.
How to Edit Cookie ?
  •  Firefox users have it the easiest:
  1. While on YouTube, use Shift-F2 to display the Developer Console.
  2. As you may know, it comes with functionality to set, modify and delete cookies in Firefox.
  3. Write or copy cookie set VISITOR_INFO1_LIVE Q06SngRDTGA and hit enter afterwards.
  4. This sets the right cookie. Reload the page and you should get the new player interface.
  5. Please note that you may need to switch to the US English interface on the site for it.