It definitely does what it says. However, just having the extension without it active causes audio desync from videos where the audio is delayed. I've done a LOT of testing to make sure and narrowed it to this extension.

It is good , works great on YouTube. Simple and straight forward BUT it does not work on all sites , for instance it does not work on IMDB and more.

Works as described...

...however: it contains code to inject iframes for advertisements, and the script loads code from the embedded remote server. I confirmed this by downloading the extension separately (right click > save as on the "add to firefox" button), extracting the contents with 7zip, and viewing the source files.

That means that while the current version might be "safe" and only embed ads, the app owner could choose to inject malicious code at any time by updating their remote server, without even needing to update the extension itself. For all I know, it already does; it'd be trivial to quietly inject credential-harvesting code only 1 in 10000 requests to fly under the radar.

It's down to a matter of trust. Do I trust this dev? I do not. I would strongly advise not installing this extension.

As todd howard said once: "It just works!"
and it really does this does exactly what it says and is literally perfect! :)

Funciona perfeitamente e ajuda muito em situações onde o áudio original está muito baixo.

Perfeito trabalho, Aliaksandr. My suggestion is here: another tab with effects, and EQ, if you desire. Perfect work, Aliaksandr. Minha sugestão está aqui: Uma outra aba com efeitos, e EQ, se tú querer.

It just works.