i have the youtube+firefox bug combo where the sound volume always is maxed out no matter what. i have tried a few add-ons before and this is the only which manages to actually have an effect on youtube videos volume. By setting the plugin volume at 10 % i obtained my wanted youtube volume (which is different as it is 44 %). i intend to up the note to 5 stars if somehow, someday the add-on volume happens to match with the actual volume result.