Fully disable Firefox network predictor

The network predictor (formerly called 'seer') makes preemptive connections to resources in a page based on cached information. It can also do the same when the user hovers over a link.

We should verify that this thing does not actually make full HTTP requests during the hover prefetch, as that could be a linkability issue. The preemptive connections to resources may also subvert our SOCKS username+password isolation.