Firefox pages do not load when monitoring. Mentions proxy errors.

Knowledge Base

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.