Firefox: HTTPS and response code 407

For those that were wondering, HTTP 407 is "Proxy Authentication Required".

I don't quite understand. If you already went to the trouble to set up a rouge SSL proxy on a network, can't you just silently inject content into the original response that would then run in the same origin context?<p>Surely I'm misunderstanding the meaning of SSL proxy. Can someone explain how such a thing works.

I have to ask, since I like playing with HTTP 418<p>HTTP/1.0 407 Boink<p>What is up with the Boink here?