a.  Ensure Volume is at a setting you prefer. 
        b.  Output/Input are most likely showing Default.  Mine did but YouTube video's still had no sound.
             Change the Defaults to the same as the Masters at the top of the window and try your audio.
             Mine started to work again at this stage.  After i confirmed everything was working again, i changed
             Mozilla Firefox Output/Inputs back to Default and it continued to work perfectly.

Interestingly, today after changing motherboards and my old hard drives after my old MB died. Windows 10 reconfigured everything for audio except Firefox. After a lot of troubleshooting using various methods with no success (as all other aspects of windows 10 audio were working perfectly), I eventually stumbled upon a solution that finally worked for me. In Windows 10, there is an app that varies individual app volume that eventually fixed my issue and i thought it best to post here for others, if they also have this particular problem. All i did was the following: 1. Do a windows search for "App Volume". This will display Sound Mixer Options. Select it. 2. In the App Volume and device preferences screen that appears, go to Mozilla Firefox. a. Ensure Volume is at a setting you prefer. b. Output/Input are most likely showing Default. Mine did but YouTube video's still had no sound. Change the Defaults to the same as the Masters at the top of the window and try your audio. Mine started to work again at this stage. After i confirmed everything was working again, i changed Mozilla Firefox Output/Inputs back to Default and it continued to work perfectly. Please note that I made sure all my OTHER audios were working first with W10 before doing this, ensuring everything was functioning properly. ie. Other browsers were playing youtube videos with sound, playing an audio music file, etc