It is an extension which I was using for a long time except for some period when youtube changed its policy or firefox upgrade. 90% of the time this worked fine and does what is promised. I recommend to those who download videos from different sites apart from youtube. However providing conversion or allowing direct download of mp3 , where ever available would make it more user friendly.There are few apps in firefox which give an opportunity to download only mp3 in a musical video.I recommend to this app that facility may be extended.