I have the same problem with VirtualBox Linux VM, was wondering what is going on, and this post comes up. I am not sure if it is the same reason. I tried:

tc qdisc add dev eth0 root netem delay 100ms

and:

printf 'HEAD / HTTP/1.0\r\n\r\n' | nc -6 rachelbythebay.com 80

returns nothing whereas

printf 'HEAD / HTTP/1.0\r\n\r\n' | nc -4 rachelbythebay.com 80

returns as expected.

In my case, firefox has no problem if invoked from command line, but "sometimes" it just hang up when invoked from script.