I've been using this for years. Tragically, there was a period where google managed to break it... but it's back and better than ever!!! Download songs directly as an MP3, not as a video that you then have to convert! Decide what quality level to download videos! Super easy interface! This is the only addon I've ever bought for firefox, and I would buy it again no question.

BIEN

Dishita seems to work constantly on this extension, in order to keep up with the changes YouTube keep adding to try and prevent downloading. YouTube makes a change, seems it's less than a day before he's figured it out and you're downloading again. I highly recommend going with the upgrade for the higher resolutions. Well worth say, a $30 donation/gift for a year to help him out. NOTE: Dishita is very responsive to support requests and problem reports. He's putting a LOT into this.

Works great!