The extension is excellent, and its new version, available on Chrome/Brave, is also very good.
Unfortunately, for some reason that the author did not detail, he is not motivated to continue providing updates on Firefox. Help out, Firefox.

Author's quote: "the version 3 will only be available for Firefox if Mozilla changes its review process because after more than 12 years publishing my extension on AMO there are things that I don't accept anymore..."

+ Probably due to a browser update, my extension had broken some important options, so I switched to another one. Now, returning to Firefox and reinstalling the old extension, I noticed that 'Boost Audio' is better in this version, even at level '10', than the updated version on Brave/Chrome. There, the audio already starts losing quality at level '3', making it unviable to use at level '10'.