An 'Easter egg" in a browser could be defined as 'hidden content,' usually in the form of a message.
The Mozilla browser “Firefox” has many hidden text and graphics message stored internally within a “dynamic library list" (the file will have a name ending with “*.dll”) that if the savvy user types in the correct command on the addressbar and hits ENTER on the keyboard, this calls up the image, program or user message.
One of the ones I enjoyed in an earlier version of Firefox, was typing in:
About:kitchensink
And you would be shown an ASCII image of what appeared to be a large stainless kitchen sink basin. This ‘Easter egg’ seems to be absent from my current version of Firefox (3.x). I think that “about:kitchensink: still worked in Firefox v. 2.x
Mr. Roboto
Here is an Easter egg that works in v3.x of Firefox that is really nice.
About:robots

This is an ‘active window’ and if you click the gray “Try Again” button on the bottom-left, you get another message…
I like the “And they have a plan.”, -this taken straight from the new (albeit imho, over-hyped) “Battlestar Galactica” TV series. Which by the way I should add that despite my being an avid fan of the original series back in the late 1970s even with it’s laughable plots and uber-bad acting, I can’t seem to get my head around the new series despite it being leaps and bounds better than the original. That shaky shoulder-mounted handi-cam thing has GOT to GO! It's just too scary and unnerving. I mean, it makes me want to puke! -It's a commercial TV series you idiots, not "The Blair Witch Project"!
And if you are epileptic, you should probably avoid “CSI:Miami” for they are the WORST show that I can name at the moment for bad use (read: overuse) of split-second splicing and faux digital toaster effects. It’s a superfluous SPFX that is supposed to draw-in viewership to 'the busy-ness' of their work, the way the great theme music successfully did for “Miami Vice.” It's just a hook. Except in the case of Miami Vice, that show was at least, viewable. Even entertaining, - for what it was.
The Granddaddy of Easter Eggs
Of Course(!) I have to mention the biggest most important Firefox Easter egg if for nothing else, to benefit the truly uninitiated:
About:mozilla

A warning about ‘the beast’ is presented when you enter “about:mozilla” which is a parody play against Internet Exploder opps, -I means Explorer, the quintessential Microsoft browser product. Various versions of Firefox (and Mozilla) use varying themes of text, but the messages are clear.
Turnabout is Fair Play
This by Firefox/Mozilla supposed to be a retaliatory poke at Internet Explorer for it’s previous inclusion of a similar Easter egg in its browser. Whereby typing “about:whatever” and then next try “about:mozilla” you are shown different results. They were apparently trying to poke fun at chief rival Netscape Navigator (a "Mozilla" product) with a suggestive BSOD, -the so-called “Blue Screen of Death.” This clearly to imply that Netscape ‘crashes browsers.’ Most of this was going on unbeknownst to me at the time as I was still learning how to use a computer.
But for IE to suggest that 'another brand' of browser causes BSOD… -that is a classic case of the pot calling the kettle black if you know what I mean! “BSOD” is like entirely an IE-thing, imho. Windows ME was the messiest, most unstable browser of its time. That is, until VISTA came along last year. Whew… what were they thinking there?! I tried typing "about:whatever" and then, "about:mozilla" in my current version, IE-7, but nothing happens. This must have been removed from the later upgrade. I have just read that Microsoft formally stopped using added 'Easter Eggs' in its programs as part of its "
Trustworthy Computing Initiative" in 2002.
Yet they are about the repeat themselves again with yet another 'broken by design' release...Windows-7 They just keep going 'round and 'round like a moron trapped in a revolving door, -repeating their errs, hoping against hope for a different outcome! Sheez!
If You Forget the Past, You Are Doomed To Repeat It…
I just read a report at PCWorld.com about Microsoft’s ‘new’ OS called “Windows 7” (formerly codenamed Blackcomb and Vienna) and early reports on it are looking even worse than VISTA! Things that worked in VISTA now don’t work in “Windows 7” it seems. Yeah, -what are they thinking, -indeed!