If you don't have any config that needs the enabled http-inspection, then it's very likely that your HTTP-inspection basically doesn't do anything. And based on your description I would assume that the problem should be somewhere outside the FWSM.
Do you see anything in the log regarding the problems?
If you really don't need the inspection (any "filter"-command on the FWSM?) then I would just remove the inspection:
policy-map global_policy class inspection_default no inspect http
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...