I think being able to change the pitch and speed independently of a music or music video is a very basic feature and still I wasn't able to find it in any of the popular firefox addons. There were some that claimed to do so, but turned out to have some bugs and/or messed up with other extensions.

This is pretty much the best one right now,
I am surprised it isn't popular.

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Elyra Labs originally made it for chrome and published a reddit post r/chrome_extensions, and I mainly use firefox, so I put up a reply requesting the firefox release too.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!