Firefox pages do not load when monitoring. Mentions proxy errors.
Firefox does not always perform properly with proxy connections.
For example some versions have had bugs in them: http://superuser.com/questions/790006/why-does-firefox-not-auto-detect-the-proxy-after-upgrading-to-version-31
So make sure you are running the latest Firefox.
If your still having issues then use another browser to capture your media.