Today I also got an answer from a German D-Link product manager. In short, he stated that present routers (e.g. DIR-857) have new chip sets that include an IPv6 firewall.
He also said, when DIR-825 Rev B was developed, RFC6092 didn't exist, and that IPv6 was only implemented afterwords as a best effort to allow customers access via IPv6.
Then he argued, that in former days when Windows XP was current and PCs used to connect to the Internet via modems, they also had to be protected by software.
He also said that operating systems like Windows 2000 and Windows XP, which Microsoft doesn't support any more for years(*) regrettably can't keep up with IPv6, that changes one fundament of the Internet.
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He didn't say anything about future firmware updates for DIR-825 Rev B, that might include an IPv6 firewall. (*) By the way, Windows XP SP3 is still supported by Microsoft up to 2014.
If you visit the german D-Link website to see the current products (
http://dlink.de/cs/Satellite?c=Product_P&childpagename=DLinkEurope-DE%2FDLProductFamily&cid=1197318677527&p=1197318958248&packedargs=locale%3D1195806663795&pagename=DLinkEurope-DE%2FDLWrapper) you'll find both DIR-825 and DIR-857, and the data sheets of both products state the same IPv6 capabilties:
"IPv6 ready
This router is ready for the future of the Internet with support for the upcoming
move from IPv4 to IPv6. It carries the IPv6 Ready Gold Logo, meaning that
it not only supports the IPv6 protocol, but is also compatible with IPv6
equipment from other manufacturers. Using a dual-stack architecture, this
router can handle routing for both IPv4 and IPv6 networks at the same time,
so you can be assured that your router is forward and backward compatible."
They do not say, that DIR-857 has an IPv6 firewall and DIR-825 has not.