Rated 3 out of 5 stars

Very nice. But there are 2 things that'd make it really useful: to add MP3 and captions, if available.
I can see .tmp (which is .mp3) apart from mp4, webm, flv, 3gp when I grab YouTube links. There are some "YouTube to Mp3 converter" apps or sites, but I prefer all-in-one solution with this extension.
There are other ways to download MP4 but not really for captions and that'd be great.

This review is for a previous version of the add-on ( 

The extension only gets download URLs from YouTube and YouTube doesn't provide MP3 downloads (it's a video site). I'll consider adding captions support. Thanks for your feedback.