The only applications that you need are:
Python 2.4 or later (to run the script!)
MPlayer (latest version with ffflv video codec and mp3 audio codec)
Lanchy Firefox extension (if you want directly open video from Firefox)
Now I will explain you how to get it working with Firefox,
First we need a Firefox extension called: Lanchy,
this extension give us the ability to open directly from Firefox Youtube video on youtube webpage (not on all webpages that have
a youtube video! you need ALWAYS the youtube video's webpage),
After you have installed Launchy (from firefox's addon site), restart Firefox, now it will ask you to insert a Launchy's button
on your toolbar (DO IT! so open Youtube videos will be very simply),
after that go on the preferences/option of this extension and uncheck the option: "search automatically programs".
Now copy the "launchy.xml" file that you found on the package and copy it on your directory:
".mozilla/firefox/YOUR_ID.default/chrome/" , after copied it, please don't forget to edit it with your myvs.py COMPLETE PATH, and add or delete oppurtune option to pass to the script.
Now restart again firefox an watch if it recognise the command (by pressing the G).
Go on youtube.com, open a video webpage, click on the G button-> Open in Mplayer Youtube Video Streamer..
It's all!
Enjoy!
http://myvs.sourceforge.net/docs.html