Unfortunately, as you noted, it is impossible to push VLAN 1 down to the FWSM.
Aside from redesigning your network to not use VLAN 1, you can try creating another SVI and routing your traffic through the MSFC before being sent down to the FWSM. So, the packet's path might look something like this:
With this workaround, you can push VLAN2 and VLAN100 down to the FWSM and still keep your hosts on VLAN1.
I believe the reason for this is simply the enforcement of a best practice. It is assumed that VLAN1 will be used for management traffic only and not need to be firewalled. It is a best practice to move your production traffic into VLANs other than VLAN1 (though certainly not a requirement as you have seen in your case).
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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...