It has some issues on moving and resizing but nothing is perfect and I appreciate your work. Ideally depending on the size of the window it should reduce the quality for the video to save bandwidth. Previously I had "Simple Youtube Popup" and another extension called "Always On Top" but your extension it's easier to use and can replace both,
This review is for a previous version of the add-on (48.2).
This is a great Add-on for Firefox. Couple of things: 1) I recommend to remove thick borders of Player window 2) Moving Player window around is slow and sometimes it is so hard to move and it sticks to mouse 3) Make it available always on top while user works on other applications 4) When I click on extension it starts playing from beginning. I recommend to start playing from the point it was playing on the main YouTube player.
This review is for a previous version of the add-on (47.0).