In general, you do not need the add-on unless you want to open the video from the browser, though you may need it for Twitter. You might be able to just copy-and-paste the location into the player. I have not installed that “Open with” add-on, and that is the way I do use Gnome MPV. (I am not a heavy video viewer.)
It is a bit off-topic, but you can get around that “Firefox only” problem by using the browser’s Extensions panel for getting the add-on as described here.