Thank you. Works well.

However, with most/all tabs muted, the "muted" icon next to each tab is taking space, it's visually noisy and it hides/replaces the site icon.

Remedy: Hide "muted" icon unless a tab is currently playing sound.

How? create "userChrome.css" file with the following content:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
.tab-icon-overlay:not([soundplaying]) {
display: none;
}
.tab-icon-image:not([soundplaying]) {
opacity: 1 !important;
}

Save that file in chrome folder right under your Firefox profile folder. To locate that, open Firefox special URL about:support and see "Profile Directory". See also https://www.reddit.com/r/FirefoxCSS.

As someone who forgets he left his speakers on, this addon has been a lifesaver.

Couldn't unmute all tabs when decided to remove the addon. Spending precious time unmuting by hand. Pretty pissed.

why EXACTLY this exists ?!?
if it add all by itself the very one site I REALLY NEED TO MUTE to exceptions ?!??!?!?!?!?

This is great for those annoying mini videos that play on sites that you visit to download a freeware program. They pop up telling you how to install the same kind of programs over and over again its annoying. So this really helps with those videos.

An essential add-on for Firefox. Congratulations on your great work.

Simple and good! Needed this very much.

needed this so bad , instagram for some reason thinks its ok to blow out your eardrums

Been using it for some months now and works as expected. 👍 Had to get used to it at first though, there were moments I thought YT has a new feature to mute embedded videos in other sites because I got no sound xD 🤦