this sould be a native feautre in every browser (not limited to just Mute/Unmute)
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If you're using Windows 10, you can click on the volume control on the right of the task bar, as shown below. It's easy, although it doesn't allow you to control tabs individually.
Potentially related ideas here:
Better accessible tab (un)mute controls
Lower the sound volume of background tabs when media with sound is played on active tab.
I wanna suggest a feature for the desktop firefox, altough given my experience so far, it's more than likely that this new suggestion, just like the previous one, will fall on deaf ears, so to speak and nothing will change. My previous suggestion about adding "about:config" to the mobile Firefox received a lot of votes, months passed, if not even a year and still nothing was done about it and the mobile browser still doesn't have "about:config". That's why I have my doubts that this new suggestion of mine will ever see the daylight but I'll post it anyway.
When using a live chat on a website (usually for remote tech support of a product), that live chat produces a sound whenever the tech support operator sends you a new message. However, that sound is always so loud that it could wake up the dead. I don't wanna change the sound volume for my entire browser, so I suggest an option to be added in the contextual menu of every tab - "Sound volume for this tab" which would bring up a menu/dialog window with a slider where the user will be able to manually change the sound volume for the given tab. And ofc that setting will have to be permanently stored in the user's profile (not in the browser cache).
Individual volume sliders for tabs
There's already individual mute button for tabs, I wonder why there's no seperate volume sliders.
(Note: similar ideas have been merged into this thread)