IT DOES WORK!!!!! Rated 5 out of 5 stars
Alright for people that are having issues with this add-on. it does work. i had an issue with the button not showing either, but i figured out what was wrong, well for me anyways. i had user script, HTTP-to-HTTPS- redirector. the problem is that the HTTPS cancels out the add-on, or that the add-on isn't reading the right site, sine its HTTPS, instead of HTTP. so what i had to do was just make sure to exclude youtube from the user script and it worked. so if you have it so it goes to https, you have to fix that so it works again. i noticed there was a problem with this add-on and https://addons.mozilla.org/en-US/firefox/addon/youtube-to-mp3/ its the same fix.
other then that love this add-on been using it for years, and i highly recommend this over anything else