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!)
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.