Don't worry, be happy that ip6tables is at least starting ;-) 
See https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=244721 and
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=236888 for more

There are more issues

1) message is caused by:

# ip6tables -A RH-Firewall-1-INPUT -p 51 -j ACCEPT
Warning: never matched protocol: 51. use exension match instead.[root@host
sysconfig]#

Note that there is a linefeed also missing


2) Looks like there is a need of a discussion with netfilter folks, why they
mean, that in IPv6 an Authentication Header can never be occur as first
transport header behind the IPv6 header


3) If one try to setup a rule using IPv6 header matching (according to the
netfilter warning message), this would fail because of missing library:

# ip6tables -A RH-Firewall-1-INPUT --match ipv6header --header 51 -j ACCEPT
ip6tables v1.3.7: Couldn't load match
`ipv6header':/lib/iptables/libip6t_ipv6header.so: cannot open shared object
file: No such file or directory

Try `ip6tables -h' or 'ip6tables --help' for more information.


Note that the kernel would support this:
# modprobe ip6t_ipv6header
# cat /proc/net/ip6_tables_matches 
ipv6header
state
udplite
udp
tcp
icmp6

This missing userspace support is already known for

FC6:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=165145

RHEL4:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=244048

RHEL5:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=244047

I really wonder why this bug can't be fixed since August 2005.

*** Bug 230019 has been marked as a duplicate of this bug. ***

Assigning to system-config-firewall.

Fixed in rawhide in package system-config-firewall-1.0.8-1 or newer.