Ok, I'm gonna show you how to download youtube videos and have the videos downloaded as mp4, so no converting for your zune

*YAY*, with only a firefox bookmark. First copy the this:
Code:
javascript:if%20(document.getElementById('download-youtube-video')==null%20&&%20!!(document.location.href.match(/http:\/\/[a-zA-Z\.]*youtube\.com\/watch/)))%20{var%20yt_mp4_path='http://www.youtube.com/get_video?fmt=18&video_id='+swfArgs['video_id']+'&t='+swfArgs['t'];%20var%20div_embed=document.getElementById('watch-embed-div');if(div_embed){var%20div_download=document.createElement('div');div_download.innerHTML='<br%20/>%20<span%20id=\'download-youtube-video\'><a%20href=\''+yt_mp4_path+'\'>Download%20as%20MP4</a>%20'+%20%20((navigator.userAgent.indexOf('Safari')!=-1)?'(control-click%20and%20select%20<i>Download%20linked%20file%20as</i>)':('(right-click%20and%20select%20<i>Save%20'+%20(navigator.appName=='Microsoft%20Internet%20Explorer'?'target':'link')%20+'%20as)</i>'))+'</span>';div_embed.appendChild(div_download);}}void(0);
then right-click on your bookmark toolbar and click "New Bookmark". You can name it what you like, such as, "Download YouTube Video". Then paste the text you copied into the location area. Then go to any youtube video. once on a page with the video, click the bookmark you just made, and you should see some text in the video description box the says "Download Video" or the like. This is what it should look like:
See attachment
you can either left-click the link(which will download as "video.mp4") or you can right-click the link and click "save link as" (where you can save it as "blahblah.mp4")
after thats done, hit save, and enjoy the video ^_^