If you are using any other Browser or don't want to install firefox add on, then there are many websites which allows you to download Videos from Youtube and other Video websites. You have to simply paste the URL of the YouTube Video you want to download into a form on their website. The websites like YOUTUBEX KEEPVID and VIDEODL allows you to download Videos from YouTube, Google, IFilm, Metacafe and various other Video Websites.
After you have downloaded/saved a YouTube Video on your hard drive, Make sure that the extension of the downloaded file is .flv (flash Video). If not, you must change the file extension to .flv (flash Video) format. The .flv is not a common multimedia file and you would need a software called FLV Player to play it. Downlaod and Install FLV Player or VLC Player into your computer to open the flash video file.
If you want to convert/decode the FLV videos to other formats like MPEG/MP4 so that you can store them onto your IPod or PSP then you can either download a free software called Riva FLV Converter or you can visit the Vixy website which offers an online FLV Converter.
Happy YouTubing!!