Try to set this pref to false on the about:config page to hide this icon.

• browser.tabs.showAudioPlayingIcon = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

Hi  !

You can disable that icon in the about:config page :

Type in the address bar about:config (press Enter) (promise to be careful, if asked) Type and look for the preference : browser.tabs.showAudioPlayingIcon and set its value to false

Note : As of the latest Firefox version media in a background tab will not be auto played until you select or focus on it.

Edit : Looks like cor-el posted (almost) the same answer while I was typing
mine ......

Hello, browser.tabs.showAudioPlayingIcon doesn't change the muted icon, it hides only the icon that shows that tab is playing when it's not muted.

I need to hide icon that tab is muted.

Setting the above mentioned value of that pref should do it.

Do you by any chance have an add-on installed like 'Noise Control'  ?

If so : would you please remove it  ?

I don't think I have any other extension installed. It's Firefox Quantum, here is screenshot:

https://pasteboard.co/GOpQRkG.png

The tabs without "muted" icon were open before extension was enabled.

And I did restart the browser after I changed this option. After restart all icons are gone, but for every tab I open they again show

Petrb said

I don't think I have any other extension installed.

Could you check, just to make sure  :

3-bar menu => Add-ons => Extensions

Petrb said

I muted all my tabs with extension

Forgot to ask : which extension  ? And : did you remove it  ?