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 do this:
- type about:config in the address bar
- locate the network.dns.disableIPv6 setting
- 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.
