How do I individually mute audio per tab in Chrome or Firefox?

Since version 42, Firefox has a native icon to show what tabs are playing sounds, and it is possible to click on this icon to mute them:

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On Chrome, type this in the address bar to enable the same feature as Firefox: chrome://flags/#enable-tab-audio-muting. After you enable it, you need to restart the browser to make it work.

For Chrome: In the address bar, go to chrome://flags/#enable-tab-audio-muting, then near the highlighted text on the resulting page, click on 'Enable'. Then, restart Chrome.

When enabled, the audio indicators in the tab strip double as tab audio mute controls. This also adds commands in the tab context menu for quickly muting multiple selected tabs.