I've been doing a lot of cross-browser testing lately for AJAX applications and Firefox has been killing me since it takes several seconds to load a single page when using localhost URLs on Windows Vista. Internet Explorer 7 and Safari have been blazing fast with the same pages so I finally took the time to research the issue with Firefox and came across the following information:
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.