I had no idea that Firefox added Media Control support, pretty cool! I looked into this and as of Firefox 81 these are enabled by default:
https://support.mozilla.org/en-US/kb/control-audio-or-video-playback-your-keyboard

The docs currently say:

On a modern Chromium-based browser, you can control Koel using hardware media keys without even having Koel or the browser focused.

It sounds like this extended to Firefox now too, but if we are going to update the docs, we should check if we can say on "modern browsers", thus including Safari, Brave, etc. Or if things should be written specifically to Firefox and Chromium-based browsers.

Once that's been determined, a PR can be made on the docs site:
https://github.com/koel/docs/blob/master/src/README.md#using-the-web-interface

Isn't it now a norm on macOS? I don't own a Windows anymore (since like forever), so I can't really confirm this.

I did some testing on macOS 11 with Safari 13 and didn't work as smoothly as in Chromium or Firefox. I decided to make a PR leaving out Safari for now, it may work but this is at least an improvement.

koel/docs#20