I recently upgraded a computer (Dell Optiplex 390) to Windows 10 version 2004. I installed the latest version of Firefox Ver 80.0.1. I have no audio coming from the Firefox browser. All of my other browsers (Edge, Chrome) have good audio as well as all my media players (VLC, MS Media Player, etc.). For example when I play a YouTube music video there is no speaker icon in the YouTube tab. I can right click, and click 'Mute Tab' and the speaker icon appears with a slash through it. Firefox shows up in the audio mixer but there is no pulsing volume level bar to indicate a signal like there is on my other browsers and media players when playing audio. I have uninstalled and reinstalled Firefox twice and even downgraded to Firefox version 79 but none of these actions restored Firefox sound. So since all of my other browsers and media programs all have good audio, I don't have a computer hardware problem or else none of them would work. Somehow Firefox is not sending an audio signal to my sound hardware. I saw a suggestion regarding the about:config entry browser.tabs.show.AudioPlayingIcon, but that entry was not present and was not present on my other computers running Firefox. This must be for an older version. Thanks for your help!