You need to check your keyboard software driver for it's configuration. Firefox has no control what those settings are since the software/driver for the keyboard isn't a Firefox Browser software. The Browser uses the key configuration the keyboard driver software install to use.

Hardware media keys requires at least Windows 8.1.

I'm going to go and install windows 11 right now or I won't be able to live... It's a ridiculous imposition made just to use the latest version of the OS. Thx for the answer.

Unfortunately that is the nature of O/S system. And if the driver/software doesn't support on Windows 11 for the media keys it's not going to work either. So you need to check with the keyboard driver support site to verify working with W11 or not as there is no info as to what kinda keyboard you have.