1. Install the
Greasemonkey Extension for Firefox.
2. Right-click
this .js file and select "Install User Script" (if you don't see that option try step one again)
3. Now on every youtube video page there should be a red bar at the top, that says "Save As" to download Flash video. (see screenshots
here)
4. Right click the text in the red bar, select "Save Link As," and save the file as a .flv file. Firefox will rty to save it as "get_video.php," you need to change the ".php" to ".flv," and change the first part to whatever you prefer.
Now you need to either download a media player that runs .flv files, or download a program that converts them to other file types. I suggest download Riva FLV Encoder:
http://www.rivavx.com/index.php?encoder&L=3
and heres how you use it:
http://forum.videohelp.com/viewtopic.php?t=259578