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Elyra Labs originally made it for chrome and published a reddit post r/chrome_extensions, and I mainly use firefox, so I put up a reply requesting the firefox release too.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