use window.dump to make firefox log to stdout

This allows outputting the trace messages to stdout using https://developer.mozilla.org/de/docs/DOM/window.dump

Useful to make firefox more active in the testing procedures described either in http://googletesting.blogspot.se/2014/08/chrome-firefox-webrtc-interop-test-pt-1.html or https://blog.andyet.com/2014/09/29/testing-webrtc-applications