600% Sound Volume (Fixed) by Savva

Control volume per tab from 0% to 600%.

Use the slider in the popup to boost quiet videos and audio, switch between tabs playing sound with one click, and use keys 0–6 for quick adjustments.

No ads, no tracking and open source: https://github.com/savvamadar/600-sound-volume

(Based on https://addons.mozilla.org/en-US/firefox/addon/600-sound-volume-privacy - used as a base, the current state of the add-on is broken)