Works out of the box and I see it updates the youtube's Stats for nerds box with the increase in volume:

Volume / Normalized
100% / 1109% (content loudness -20.9dB)

Current version as I write this, it's working. Install, pin to Toolbar, click the new white puzzle piece icon on your Toolbar, there's some settings, I'm using the default:
1) Amplify Audio: Yes
2) Adjust to peak: No.
It just works. Thumbs up!

Finally fixed the problem of videos with different volumes being disruptive.