In order to use the extension after adding it, you just have to press the icon and a popup will appear that will list the name of the tabs and the volume they are currently at. Accompanying sliders will appear for each of the tabs of the current window which can be adjusted in order to change the volume of any tab.
Note: Only works on websites that use <audio> or <video> HTML elements. Websites that don't use these to playback media will not work.
If the extension doesn't seem to work the first time installing, try closing all tabs and reopening Firefox.
You can get into contact with the developer through the extension's GitHub page.
