Click the Padlock icon on the left of the address bar
Enable Autoplay to Allow Audio and Video
Enable Autoplay, Camera, and Microphone.
If it doesn't pop up, open your Firefox preferences, go to Privacy & Security and scroll down to Permissions. Make sure that it allows Demodesk to access your Camera and Microphone:
If this does not solve it, then probably your system is blocking permissions. Go here to solve it.