No, that's just to do with retrying automatic proxy configuration.
Is 408 the most appropriate status to be sent here?
408 Request Timeout
The server timed out waiting for the request. According to W3 HTTP specifications: "The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time."
That isn't strictly true (in my circumstances anyway), the client DID produce a valid request but the server was just too busy to respond in the required time.
I would have thought this should be a 503 - to which I would presume Firefox wouldn't automatically refresh the URL.