This solved the issue for me...Right click the speaker icon in your taskbar and choose Open Sound Settings from the menu. Scroll all the way down to Advanced Sound Options and click on App Volume And Device Preferences. You'll get a pop-up window with relevant apps, Firefox will be among them. After a system update the choice of audio devices Firefox accesses may not update correctly; for instance, it could either show default instead of choosing the correct audio device among options, or it could have selected the wrong audio device among options. Make sure the correct audio device is selected. :-)