Rated 1 out of 5 stars

"This add-on is not compatible with your version of Firefox".

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

Normally it works well.
I just tested on 38.0.1 W7.

Only *sometime*, when there are a lot of tabs opened, it may happen to not pause, maybe because FF slows down with the mem, but this seems a FF problem, not my addon one, as normally it works well.
Another known problem is when you launch a YouTube page from the bookmarks with CTRL pressed: even in this case, only *sometimes* it doesn't pause on background, but that's a problem of Youtube+FF javascript handling the page "document.hasFocus()" property: I can't solve this at the moment. YT sometimes responds the page is "true", in Focus, even if it's not...

If you still have the background tab problem, be more specific, as to reproduce the problem, so I can give more help: are you using Flash or HTML5, open new tabs from a YouTube page with ctrl, how many tabs opened, etc

Anyway, of all the features of this addon, for only one not perfectly working only in your environment, you give a 2 star rating?? Not fair, please reconsider it!

1) The only thing I can suggest you: create a new FF profile and then install only this addon and see if it works. I suspect it could be some other addon or greasemonkey script interfering.
Aslo, obviously, check the checkbox relative to AutoPause is on.
2) I've not tried vk.com, but as it's impossible to adapt the code for every site, I inserted the option to deactivate the addon on embedded videos, so check out the relative checkbox and you will not have any other problem with that.
Another important thing is to use an addon that blocks the ads in YouTube videos: my addon can't block the ad at the beginning, so the video remains in play! I suggest the good adblock plus.