If you already have icmp any any, do a logging buffered debugging on the pix and do a show log after you ping to see if the pix is denying it or some other acl on the next hop router. It would be a good practice to do a clear log before you do the test.
Make sure you are also trying to reach hosts that have translation, even specifying in the pix the same address or no translation for the inside hosts.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...