[karatchov]

Ive been using Openwith on firefox and chrome, and it works flawlessly

It needs a launcher (aka native host client) to start the 3rd party app, in this case mpv with the correct arguments.

The launcher is technically only started (then immediatly terminates) when you trigger the openwith action, so no memory/cpu overhead.

The official openwith solution is a python script (requires python), for windows there is a native and lightweight launcher: owclauncher

With this setup, you can launch any action with a simple right click: play in mpv, play in low resolution in mpv, download video/audio with youtube-dl, play in 2nd screen ...

[ftk_]

There's also play-with extension which doesn't require native client. https://addons.mozilla.org/en-US/firefox/addon/play-with/