I'm not sure what the performance impact of that is, since that function is called for every request the browser makes. Not resolving DNS was a deliberate choice, and is why the option text mentions LAN IPs. It's fast (since we're just doing a simple pattern match) but imperfect.

It might be that it doesn't make much difference if the lookup is always cached internally by Firefox already, but the resolve documentation is a bit light on detail, so I don't know if we can rely on that or not.