Do you mean the sound icon that appears when a tab is playing sound?

Note that you can also find "Mute Tab" in the tab context menu.

That icon is only displayed when you are on a web page that plays sound.

You can also check this pref on the about:config page.

• browser.tabs.showAudioPlayingIcon = true

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