Yesterday as I was browsing for a new Firefox theme (I ended up with Rein, I came across a screenshot which showed a very unusual screen from my favourite browser. Looks like there are some hidden "eggs" behind the "about:" URL prefix.

For example, just try to type in the Firefox address box the following addresses, marked in bold (e.g. "about:robots" - without quotes):

• about:robots - displays a funny message and an odd button; I have no idea if it has any use (the button doesn't seem to do anything), or if it's just reserved for now.
• about:mozilla - displays a red page, with a white quote in the middle. The quote does have some subtle references, but I'll leave those details to the viewer.

There might be more, please feel free to leave a comment if you know others.

Have fun!