Looks like its working for the most part.
But you can not see the progress of the downloads.
So you can not cancel a download or see how long it will take. (they dont show up in the about:downloads tab)
I chose this addon because my favorite addon for youtube downloads is not being upgraded to ff57.
I really dont like the web extensions only update mozilla forced upon us!
But i appreciate the effort the maker of this addon is making for us.
I hope these points will get fixed at some point, and this could become my new favorite addon for youtube downloads.