Can be turned on by setting preference
network.websocket.override-security-block. Websockets can only be used if
override-security-block and network.websocket.enabled are both set to true.
At a future time, with a more secure websocket protocol, the
override-security-block preference can be removed.
This action is based on the security concern over an HTTP cache
poisoning attack as described in
http://www.adambarth.com/experimental/websocket.pdf
