Here's how I was able to get the wonderful tool mtr to go through my firewall:
asa-gw(config)# class-map icmp-class
asa-gw(config-cmap)# match default-inspection-traffic
asa-gw(config-cmap)# exit
asa-gw(config)# policy-map icmp_policy
asa-gw(config-pmap)# class icmp-class
asa-gw(config-pmap-c)# inspect icmp
asa-gw(config-pmap-c)# exit
asa-gw(config-pmap)# exit
asa-gw(config)# service-policy icmp_policy interface outside