How to fully disable sound in Firefox?

Does anyone know how to disable sound for Firefox permanently?

I used an extension called "muter," but the problem with it is that after I close Firefox and open it again, sound is playing again until I click the muter to mute. And even though the muter extension has the option to Remember the mute and stay muted, that option doesn't seem to work unfortunately.

Is there any way to completely disable sound in Firefox?