It doesn't adjust the volume site wide. It only sets the volume for individual pages. :(

It should not be free as some pay apps do nothing as they promise and this one does!

Sorry, but the Firefox Task Manager (about:performance) says this addon has high CPU usage when it's not doing anything (no volume actively being adjusted)

works fine on all websites except Youtube. The site runs fine if I don't use this add-on. When I do, it buffers then shuts the volume down when the video starts to play again.

es muy bueno me aumento por completo el volumen sin distorsionar el audio

Something I never get to say: It does what it claims to do in one click and one drag. It has no frills, no extra clutter, it's exactly what I went looking for.

Update: To anyone who says it isn't changing, make sure that you're on the tab that you want to make louder/softer and that the media is actually playing when you move the slider.