Starting in Mozilla Firefox 81, you can enable or disable the media.hardwaremediakeys.enabled flag for using media keys on your keyboard to control the active media session.
You control media (audio or video) playback inside Firefox without clicking on the video or audio itself. You can now control it by pressing the hardware control buttons on a keyboard or headset, pressing the button on the virtual control interface or by ending commands via specific protocol, such as MPRIS.
For example, if you press the Volume Up, Volume Down, or Mute media keys on your keyboard, you will see media controls (Reverse, Play/Pause, Forward) next to the volume control on your desktop that you can use to control the active media session in Firefox.
This tutorial will show you how to enable or disable the Hardware Media Key Handling and volume control feature in Mozilla Firefox for your account in Windows 7, Windows 8, and Windows 10.