Works great for downloads but it stopped joining audio and video for youtube videos a long time ago and it hasn't been fixed since. There are a few workarounds that are all a total pain.

I really miss being able to download audio files as .OPUS!

GooooD

Firefox 48 and 52 - does not work
Mobile Youtube - error

Do you want to download some garbage to your computer for this app works?

Works as advertised, but the MP3 converter function is garbage. all the audio I was getting after conversion was peaking and full of static. Ran it through ffmpeg locally and it sounds fine.

用了这么多唯一可以下YouTube视频的插件，嘿嘿~

You can download the video and also convert it to audio only.

Older version worked for me, but 1.0.8 does not work for me. So I rolled back to 1.0.2.

I was trying to dowload rhymes and some baby songs from youtube and this is by far the most simple and effective extension an di have tried most of the other ones but nothing beats this one. well done devs you made my life easy