Sound not dimming on Firefox while using Google maps.

Hi all. I use Firefox as my default browser and I listen to different radio stations that have no app available (they only broadcast over web). While playing them over Firefox, if I happen to have google maps open for directions, the sound on Firefox won't dim at all whenever Google maps is giving me any instructions and both sounds (Firefox and Google maps) will overlap. This doesn't happen with other apps like Spotify, which dims the sound perfectly fine with every interactions from Google maps. I've busted my brains trying to figure this out unsuccessfully. You guys happent know how can I fix this, please? Ty!