(adsbygoogle = window.adsbygoogle || []).push({}); Hey!
There's an add-on called Video-Download Helper!

Link:

    

https://addons.mozilla.org/en-US/firefox/addon/3006/

What to do after this.
Navigate to any video site (i.e YouTube, Google Video, Yahoo Video ect ect)
Once you load your video, you should see these three balloons colored Red, Blue and Yellow. When you see those turning, it means that you can DOWNLOAD that video.
Now, press the little arrow to the RIGHT of the balloons, and press the video file.
It'll pop up a command prompt asking you to save it. From there, rename it with the extension you want (.mp4, .mp3, .mpeg ect ect)
Thanks for reading!

Lewy!

or anyone can download real player and download any video on the internet

real player is cool but most videos are flash format and needs converted but real player has a converter luckily

There are also websites I believe that allow you to download the FLV files from youtube given the URL of the video.

Or that youtube downloader from download.com

I have video helper. It's badass

or you could use vidtomp3 converter, u just need the video url and copy it onto the site and bam!, free music