I'm bringing a problem, but I'm also bringing a solution. At least the solution that worked for me. There are at least a couple Mozilla threads on this, but they didn't solve my problem. If you have no audio in YouTube, or some particular Firefox tab, go up to the specific tab that you're not getting audio in. As you move the mouse to the Red play button which is YouTube's logo, if my fix is the right fix for your particular lack of sound, you will see it turn to a Muted speaker icon. Left-click that muted speaker so that it unmutes! To reiterate, evidently Firefox has the ability to mute or not mute a single tab! Also, you may see the word MUTED in the area immediately below and to the right of the Red play button, helping you to know what's going on. The thing is, the Muted speaker icon ONLY appears if you've Mouse'd over it, which I think is suspect programming.