this is the easiest way, its merely a bookmark you click in firefox and BAM theres an instant link in youtube to download as MP4. drag the "get youtube video" link on this page
http://googlesystem.blogspot.com/2008/04/download-youtube-videos-as-mp4-files.html
to your link bar underneath the address bar. then on any youtube video page, just click that button and a link will appear by the video information.