This will work in Mozilla Firefox:
Make sure you have the latest updated version ( Firefox/3.0.3 ), click Help, Check for updates.
Once updated, make sure that the Bookmarks Toolbar is showing : view, Toolbars, Bookmarks Tool
Copy the following:
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'&fmt_map'%20+%20swfArgs['fmt_map']%20+%20'&t='%20+%20swfArgs['t'];
Right click in the Bookmarks Toolbar and choose New Bookmark.
In the box labelled Name type Video Download.
Right click in the box labelled Location and choose paste.
Click Add.
Go to youtube and find a video you want. Play video and click on the Video Download Bookmarklet you created. Choose save and location to save as required.
Credit for the excellent code: Lawrence Albert at
Unlock for This.