How can the page know I'm analyzing it with firebug

Question

Look:

!

Wow!

How can the webpage know I'm using firebug?

BTW I couldn't find out how to show the translucent add banner.

Answer 1

You can do:

if(window.console && window.console.firebug)
{
    alert("Firebug active!");
}

Answer 2

This works not only from the Firebug console, but also from within the page:

if (document.getElementById('_firebugConsole'))
   alert("fire!");