This right here, OP. On my phone it’s
Settings -> Security & privacy -> Privacy Controls -> Permission manager
I bet if you remove FF’s mic permission and then open it you won’t see the icon anymore. My phone will put a little green dot in the upper right for an app that has permission to access location.
Well… hm. Now I can’t manage to make my FF show the Mic privacy icon no matter how I set the permissions.
Mine are always shown inside the green background before it fades to the little dot though, if yours does that stock behavior with the permission icons and this is a different icon then I really don’t know.