Keep Quiet Utility од G Polites

Once installed and enabled via the toolbar button, Keep Quiet will automatically mute all new tabs and windows. If a particular tab contains audio that is desirable to listen to, clicking the speaker icon on the tab will allow the content to be played.

Please note that since I first released this add-on, others have stepped up and developed new sound control utilities with richer feature sets that I never had the time to add myself. I will update this plugin if it simply stops working in a future release of FF but I would recommend the others if you desire more flexibility.