I would give this project a try Skype Player. It works but seems have the same problem (at least for me) that stereo mix would give, so here is a work around if that happens to you. I would first try it stand-alone though.
You will need.
2x skype accounts and the ability to run them simultaneously see this link
The first skype client will be used as your jukebox
- you must allow skype play access
- set the audio input -> a dummy input; for example you could just stick a jack extender into an unused analog mic port or line in.
- set the audio output -> a dummy output; for example I use my Digital Out as my main output, but have nothing connected to my Stereo Analog Output and so I selected that.
The second skype client is what your going to use to listen and talk
- you must deny skype play access
- set your audio input -> headset microphone
- set your audio out -> headset speakers.
Open skype play and set balance all the way to Playback.
Under playback devices make sure that your dummy input is selected as the default communication device as well as the default device (otherwise skype play might echo)
To the left is My actual account id be talking with and to the right is the one that is streaming music.
EDIT: Oh I see you want it to be streamed from firefox. In that case you would set a dummy output device (analog out for example) as your main output device in the playback preferences and a dummy input device as the main input device. In your music skype client you would set the input device as stereo mix and the output as the your headphones make sure to turn it down to zero though and/or mute that skype client in the sound mixer along with any other sounds you don't want to be streamed. Keep the second client the same as described. You wont need Skype Play.