(An Easter egg is an intentional inside joke, hidden message, or feature in a work such as a computer program, video game, or movie.)
Currently, not too many exist, but I may find more in the future.
Step 1: About:robots
If you search Firefox for about:robots, you’ll see this fun pop-culture-laced message.
Step 2: About:mozilla
If you search Firefox for about:mozilla, you can read the gospel of Mozilla.