Basically the title. The new feature in Firefox 42 where you can mute the tab from a little icon isn't showing. While in the tab I can hit control-m to supposedly mute it, but this does nothing other than briefly show a little muted icon in the tab. I'm running Ubuntu 14.04
Sadly, Firefox makes it possible to style that icon.
I watch some Televisions on Internet which tend to show advertisments as follows:
1) They style the icon to not show up, some starter pointers about doing that may be seen here.
2) They purposedly raise the volume during ads by up to 200%.
3) They disable the volume on the video player.
4) They disable the ability to pause or stop the video player.
It's very intentional, in the first weeks past the mute button release I could silence those very noisy ads by clicking the mute button on the tab, now they just remove it so to force to listen to the whole ads.