Dan Wahlin found out why the Firefox is responding so slow when it tries to connect to the localhost which for example is being used by the Visual Studio 2005 and Visual Studio 2008 integrated webserver. He writes:
It turns out that the slowness is caused by an IPv6 issue with DNS and can easily be resolved by turning IPv6 support off in Firefox while doing localhost testing. To make the change, type about:config in the address bar, locate the network.dns.disableIPv6 setting and double-click on it to set it to true. This does the trick for the Firefox localhost issue on Vista and everything is running fast again.
Good job, thanks Dan!
Source: Fixing Firefox Slowness with localhost on Vista.