Firefox add-on for youtube

Hi

I want to download mp3 and HD video from youtube. Please suggest me a best firefox plugins for doing the same.

Thanks in Advance

I don't know what is best but I like youtube downloader http://youtubedownload.altervista.org/, Riptiger http://www.riptiger.com/, and Keepvid http://keepvid.com/. They are not FireFox plugins but they work
very well. Riptiger is not free but it can do some things the others cannot, and, its fast.