The experimental add-on YouTube 2 MP3 will convert and download YouTube videos in the .mp3 audio format. There is no doubt that YouTube has a large collection of music videos, so this type of service was bound to happen eventually.
Once installed, the add-on will add a link to each YouTube video page that says, “Convert & Download as MP3”. Once clicked, the extension takes you to another web site where the mp3 will be available for you to download. The only downside I could find, is that it takes some time to convert the videos to the .mp3 format.
As a bonus, if you want to use the YouTube video to mp3 converting service without the add-on you can do that too. Just bookmark this link: http://youtube2mp3.wmn.eu/
You can download YouTube 2 MP3 extension on the Firefox Add-ons web site.