Recently I tested all audio addons, which were updated during the last 12 months. Capo by Elyra Labs is the only one that boosts Spotify. It offers the best sound (by far) and works with Firefox exactly like it does with Chrome. The bad: Capo doesn't catch audios embedded in Yandex Search. It is too heavy (1,5MB) for unstable live streams. The code is minified, not easy to check. Since version 2.6.10 the addon implements full data transfer to capotune.com. Long story short: Capo is a semi-professional special interest tool, not recommended for every day usage.