I'm not sure what happened, maybe a firefox update or maybe the switch to VS 2008 Beta 2, but recently I have noticed that Firefox will take literally 1 whole second to download each and every script, stylesheet, image, etc, from the Cassini web server when I'm debugging ASP.NET stuff on my local machine. Really annoying, and a waste of time.
So, suspecting it had something to do with a bug in VS 2008 cassini I googled "visual studio 2008 cassini" and came up with Dan Wahlin's blog post about what was causing this (thorugh Mike Ormunds post).
From Dan's blog:
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.