HTTP Proxy

If you want to use the "Browse Activity" in a network where you can only access the internet through a proxy server open the browse activity and naivigate to:
about:config

There you can type proxy into the filter box so that only options regarding the proxy server are shown. There you can enter ip and port of your proxy and then you should be able to access the internet.

If you use an HTTP proxy, you also have to set the proxy.type to "1" to tell the browser to use that type of proxy.