Doesn't work very well.

I had the 'pause video in background tabs' selected, but then when i would click on that tab, the video was still paused (i did not have the 'pause foreground tabs' selected), it doesn't autoplay. (and no i don't have autoplay disabled)
Also the whole feature itself is pointless since videos opened in background tabs are paused anyway, i think it's a feature youtube itself implemented at least 6-12 months ago (at least on Chrome)(to save bandwidth and to make sure you watched those ads) and then they autoplay when you click on them, just as they should.

When the video ended it would loop, even though i did not have that selected.

Then when i came here to write this review i saw that the +Add to Firefox button is still blue, so i clicked it, which i guess uninstalled the "old" one and reinstalled it ?
Now the video doesn't loop anymore, in fact i can't find the loop setting anymore in the settings (it's still in the toolbar), so maybe it got an update.

The default volume level setting is fickle, sometimes it works, sometimes it doesn't. Which is a shame cause that is it's best feature.

The description to the Remove Ads button says that right clicking on it disables Annotations, this again doesn't seem to work, or at least it doesn't toggle the setting in youtube and i couldn't find a video with annotations to test it. Also you can just disable annotations in the youtube settings, not sure why this is even needed.

In fact i'm not sure if the whole extension is even needed, it just doesn't do many useful things, for me anyway, and what it does do, doesn't work consistently, how this got featured is beyond me.

Also the +Add to Firefox button is still blue, does that not change to a green "Already added" button ? does Firefox/Mozilla not have that ability ?