Dose not work.
Each slider can move but sound is not changing at all.
Other Equalizing addons for Firefox dose also not work.
But same addons can work in chrome.

1 of the Top 5 Browser Extensions of all-time...

I use this extension because at times I bumped in a video with only mono sound. It's annoying. With this extension I can click the Mono flag and the audio will play on both sides. You can also change the actual equalizer which can also be cool. :-)

удобное спасибо

Works well. I needed to refresh the tab YouTube was on after installing, but no big deal. It really helps me correct a number of audio problems on videos such as too much/too little: bass; treble; mid range; quiet voice overs; background noise and such. Thank you! :-)

So far, so good!