Firefox 3.1 adds Private Browsing
Firefox 3.1 will come with a Private Browsing feature that can be activated through the Tools menu.
Not sure how useful this function really is – all it does is not store traces of your browsing-history on your machine. It doesn’t keep you anonymous from websites or your ISP, and is definitely not meant to protect you from spyware that try to intercept your traffic.
This can also be activated on startup by going to about:config, and toggling: