Maybe have a look at these:
https://addons.mozilla.org/de/firefox/addon/aria2-integratio...
https://addons.mozilla.org/de/firefox/addon/aria2-integratio...
The second one needs the "native client" which I mentioned above. It uses effing python to pass the link from the browser to the player! It's a workaround and an ugly complication. The more I use technology the more I come to despise those levels of software strata solving the problem the lower layers introduced for disputable reasons.
Yeah, it's unfortunate that it needs the Python (or Ruby) script as a shim to MPV. But it's not an inherent requirement: the extension could execute MPV directly if MPV understood the weird length-prefixed JSON protocol that Firefox and Chrome use.
The aria2 one didn't have an update since 2019 though.