How can I detect which setting in the OS X could block
IPFW, LittleSnitch no such rules?
> cat /etc/hosts
The obvious choice would be if stealth mode were on. That blocks pings to localhost on all my Mac OS systems. Go to System Preferences, Security & Privacy, Firewall, Firewall Options...
Also, if you have "Block all incoming connections" that might also affect things. I would certainly turn off LittleSnitch since it also can and will block that sort of traffic if you haven't allowed it.
Most likely it's your Mac OSX Firewall.
Go to System Preferences / Security & Privacy / Firewall and see if it's on. If it is, turn it off for a moment and see if you can
ping localhost (please note, before you can turn the Firewall off, you'll need to click on the padlock in the bottom left corner and authenticate).
Don't leave your Firewall off though, unless you're absolutely sure that your Mac is not exposed to any external traffic at all (i.e. it's really only safe to do that when you're in a completely isolated network). The procedure above is to verify that it is the Firewall and not something else that's blocking the ICMP traffic to localhost.
It could also be a problem with the text-encoding of the /etc/hosts file: https://superuser.com/questions/541961/mac-os-x-mountain-lion-cant-ping-telnet-localhost-point-to-a-wrong-public-ip