first open the video you want to download click the hq tab under the video and then post this script
javascript:window.location.href%20=%20'http://youtube.com/get_video?video_id='%20+%20swfArgs['video_id']%20+%20"&fmt=18"%20+%20"&l="%20+%20swfArgs['l']%20+%20"&sk="%20+%20swfArgs['sk']%20+%20'&t='%20+%20swfArgs['t'];
on the address bar and then wait for the save video file and then voila! you have your HQ MP4 vid...:p