To install it, just drag this link to your bookmarks toolbar, or copy the link and make a new bookmark and paste the link content there. Here is the actual link content:
javascript:var url=document.location.href;if(url.match('http.?://(www.)?youtube|youtu\.be')){yid=url.split(/v\/|v=|youtu\.be\//)[1].split(/[?&]/)[0];window.open("http://www.youtube.com/v/" + yid + "&hl=en&fs=1");}else{alert("Could not find video ID")}
(you must include the "javascript:" part)
To use it, just click the bookmark when you have a Youtube page opened.
Enjoy!