There are some moderately amusing Easter Eggs in Firefox 3.  Simply type one of the following into the address bar (no spaces or http) and you’ll get a message:

about:robots
about:mozilla
about:config

(ok, technically “about: config” isn’t an Easter Egg, but the title on the warning screen is relatively amusing!)

Enjoy!

Also, don’t forget to check out:  Firefox 3 Super Speed Increase - speed up your Firefox with just a couple of changes to the default firefox settings.