From where does Firefox get the list of audio input devices that can be shared?

Hi, On my Debian wheezy machine, I'm normally able to use my built-in microphone and web camera without problems (my sound card is a Realtek ALC269). In Firefox (Nightly), the audio output works like a charm, but when a website makes a WebRTC' GetUserMedia request, the prompt for sharing devices lists apart from "default" seemingly just a list of all (virtual) sound output possibilities, none of which works (other programs, like jitsi, mainly just display a "default" option, which works out of the box). The webcam, on the other hand, works like a charm. I get no errors. The problem is the same for Firefox Nightly, Iceweasel and Chromium. Any help would be appreciated. Regards Thomas