I recently deployed IPv6 on our network. I know it is working properly because new installs are able to route via IPv6 OK as well as 99% of the existing installs. That being said, I'm unable to route to IPv6 sites using either IE or Firefox on my XP workstation. I can ping6 the IP in question fine (ipv6.google.com in this case) and traceroute to it OK from a command prompt. I can also telnet into port 80 of the IPv6 IP from a command prompt.
I was able to route using IE or Firefox at one point on this workstation during the deployment but I can't any longer and I'm not sure what, if anything, that I changed on the XP workstation. This morning, using VMWare on workstation in question, I created a virtual machine and installed a new copy of XP. I am able to use the browser to route to IPv6 sites using this virtual XP machine running on the XP workstation. I also installed a Vista virtual machine and that worked too.
On the workstation that is having problems, XP is up to date with all the latest patches. IPv6 has been uninstalled and reinstalled. I checked the about:config of Firefox. IE is the latest version. Can ping6/traceroute to IPv6 IPs, just can't bring them up in a browser. I sniffed a working setup and the workstation that isn't working and the only difference in the packet sniff is the GET / HTTP/1.1 is never sent from the non-functional workstation but they both have a successful three-way TCP negotation before hand.