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.

This extension is perfect for learning songs - it lets me slow down the music, loop tricky parts, and practice lyrics at my own pace.

👍 Very handy for A-B repeat. I'm using it to memorize songs stanza by stanza.

This has been exactly what I didn't know I needed. As a casual and somewhat professional singer, I didn't even know I need stuff like this to adjust to the song.
Elyra Labs originally made it for chrome and published a reddit post r/chrome_extensions, and I mainly use firefox, so I put up a reply requesting the firefox release too. Lo and behold! Here's the firefox release.
Now for the extension, it works on both chrome and firefox flawlessly for me, and I generally don't do reviews but I just felt I had to for this. Thanks for this wonderful extension man!