You'll only have to install the add-on and restart the browser. From then on, you'll see a new button next to youtube videos when you access them. Click it and you'll be downloading the file as mp3. That's all.
As usual, you'll be able to configure video and audio settings, because you can also download videos as video.
In short, Youtube2MP3 is an add-on for Firefox that will make easier to download youtube videos as video or audio files.