First things first. type 'about:robots' and 'about:mozilla' in FireFox address bar to get some strange pages. Firefox 3 have these two hidden pages and may be more but yet to be discovered.

Try typing 'about:robots' and you will discover this funny page.

This page has got title - Gort! klaatu barada nikto! The title is some old science-fiction movie (view more information). This hidden page has some robotics commandments :

• Robots may not injure a human being or, through inaction, allow a human being to come to harm.
• Robots have seen things you people wouldn’t believe
• Robots are Your Plastic Pal Who’s Fun To Be With.
• Robots have shiny metal posteriors which should not be bitten.

Interestingly, when you press 'try again' button you get another button saying, "Please do not press this button again."I bet even after this warning message, most of you have clicked that button.

Type "about:mozilla" in FireFox address bar and you get to see some writings from Mozilla book.

Mammon slept. And the beast reborn spread over the earth and its numbers grew legion. And they proclaimed the times and sacrificed crops unto the fire, with the cunning of foxes. And they built a new world in their own image as promised by the sacred words, and spoke of the beast with their children. Mammon awoke, and lo! it was caught but a follower.

from The Book of Mozilla, 11:9
(10th Edition)

Since Firefox is open source browser developed by the Mozilla Foundation, its contributors must have created these pages for fun. Easter eggs are made intentionally to surprise often containing bonus or hidden feature.