The question may seem disputable but still I ask him.
How all the same right must start chain firewall rules:
1- drop invalid->allow established->allow related
2- allow established->allow related->drop invalid?
Met a lot of variations of firewall rule sets, sometimes the same but with a different order of these rules.
Ask for advice. Since the basic firewall router default the second option, but for example here in http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/Filter the first option.