See What to do if Firefox won't play any sounds and Fix common audio and video issues

You can click the Volume icon on the Windows Taskbar near the clock and click the Mixer link at the bottom to open the mixer.
You can adjust the volume level for each application in that window, so make sure that Firefox is not muted or set at a low level.
You will only see the Firefox program in the Mixer if you are on a website that plays sound.

You can also try to clear the Flash cookies (LSO)

Flash Website Storage Settings panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

See also Flash Local storage settings:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help02.html

Many thanks - fixed it in one ! I wonder how I turned the Firefox sound off, but this certainly got it back on again

The issue might sometimes occur with sound mixer for firefox application is not muted or at its bottom position.

Visit the below link for more information:

http://nosound-troubleshooting.blogsp.../no-audio-in-firefox-in-my-computer.html

I tried all of these things... Updating Flash, clearing the Flash cookies, clearing the cache, updating Firefox... and I still have no sound in Firefox. Nothing is muted via Flash when I'm on websites like YouTube and Tumblr. I'm using Windows XP, and everything else seems fine. Sound works in other programs like VLC Player and iTunes, so I'm not sure what's going on.

Help, please? I'm usually the one in my family that can actually fix this kind of thing, so I'm feeling pretty frustrated.

Try restarting Firefox with the headphones plugged in the computer