• mpv < https://mpv.io >
• youtube-dl < https://rg3.github.io/youtube-dl/>

and Python probably

You need to copy mpv.json and mpv.py so the addon can use the Native Messaging API

mpv.json is a manifest to tell Firefox where the python script is located.

mpv.py is a dumb script that just launch mpv with some args

I made a small bash script to make things easier :

git clone https://github.com/vayan/external-video

cd external-video/app

./install.sh

see https://github.com/vayan/external-video/issues

Available on Firefox: https://addons.mozilla.org/en-US/firefox/addon/external-video/