chaudev wót Andrey Galysh

To correctly use the extansion media.setsinkid.enabled configuration option needs to be set to true. Navigate to the about:config, find the media.setsinkid.enabled and check it or toggle it value to be true.

After that under the addon icon or in add-on options page it will be possible to choose output device to play sound from all audio/vide HTML5 elements in all tabs.

Extension also asks for mic access on every tab/page with audio elements, this is firefox limitation, and probably will be fixed in future. Extension doesn't use mic and doesn't record anything.