Just a heads up, the demo wasn't working for me properly using HTTPS everywhere as well. The music would play, but nothing aside from the progress bar/slider would show in the white box, and the full demo never got past reticulating splines.

The issue went away after disabling HTTPS everywhere and restarting FF.

FF 27.0.1 on Win7 x64.

The demo is still broken for me. Using FF 28.0, HTTPS Everywhere. All I see is a red line on a white background when I hit the play button.