Please check the logs in sys-net, using dmesg and `journalctl -b` - look
for any wifi related entries.
What WiFi adapter are you using?
Did you make any changes to default install?
Do you have this problem in sys-firewall, as well as in other qubes?
Try to isolate the problem, by using an IP address instead of a name, in
firefox.
http://50.18.200.106