In Firefox I can right click any tab and mute it, even if it isn't currently playing audio. This is useful for websites that I know will be playing audio soon -- I mute them before they start.
Also, in general, everything I can do to a tab should probably be surfaced in that right-click menu. Having the mute button be in the address bar was not obvious to me, and it disappears when there is not active audio, whereas I want to mute the tab before the audio even starts.
