Code:
javascript:if(document.location.href.match(/http:\/\/[a-zA-Z\.]*youtube\.com\/watch/)){document.location.href='http://www.youtube.com/get_video?fmt='+(isHDAvailable?'22':'18')+'&video_id='+swfArgs['video_id']+'&t='+swfArgs['t']}
When you paste the javascript code above in the Address Bar (Firefox in this case), you can download the MP4 version of the video in the youtube. MP4 is much better in video & audio quality compare to flv and can be play in the ipod. If you see the HQ button enabled in the video, then you can download the MP4 version of the video.

By the way, you also can download the High Definition of the youtube video (if the video have the High Definition (HD) button enable) by using the script below.

Code:
javascript:if%20(document.getElementById('download-youtube-video')==null%20&&%20!!(document.location.href.match(/http:%5C/%5C/%5Ba-zA-Z%5C.%5D*youtube%5C.com%5C/watch/)))%20%7Bvar%20yt_mp4_path='http://www.youtube.com/get_video?fmt=22&video_id='+swfArgs%5B'video_id'%5D+'&t='+swfArgs%5B't'%5D;%20var%20div_embed=document.getElementById('watch-embed-div');div_embed.innerHTML=div_embed.innerHTML+'%3Cbr%20/%3E%20%3Cspan%20id=%5C'download-youtube-video%5C'%3E%3Ca%20href=%5C''+yt_mp4_path+'%5C'%3EDownload%20as%20720p%20HD%20MP4%3C/a%3E%20'+%20%20((navigator.userAgent.indexOf('Safari')!=-1)?'(control-click%20and%20select%20%3Ci%3EDownload%20linked%20file%20as%3C/i%3E)':('(right-click%20and%20select%20%3Ci%3ESave%20'+%20(navigator.appName=='Microsoft%20Internet%20Explorer'?'target':'link')%20+'%20as)%3C/i%3E'))+'%3C/span%3E';%7Dvoid(0)