hello DragosS, as a creator of a video chat this isn't yet possible unfortunately. however this is a wanted feature and is worked on for the next iterations of the service. your partner invited to the chat link link can choose the preferred webcam/microphone when he joins the conversation though...

DragosS said

I cannot find the options to change the default mic or default webcam in Firefox Hello. I am using FaceRig ( http://store.steampowered.com/app/274920/ ), so I'd like to use the FaceRig Virtual Cam DirectShow filter as a virtual webcam for Hello , just like I do in Hangouts or Skype.

I have the problem too, I need to choose the camera and mic, because I have many virtual cams, and many microphones conected.