Just a quick note for myself (and also for those of you who might be looking for a solution to a frustrating problem): If you are developing with the Cassini web server on Windows 7 and notice *major performance problems when using Firefox to load the local site, try out this fix. It worked for me!

http://joshclose.net/?tag=cassini

EDIT: The link above is down at the moment. Here are the instructions:

1. Open Firefox and type “about:config” in the address bar.
2. Use the textbox at the top to filter on “v6.”
3. Double-click on the “ipv6″ setting.