Firefox’s user interface is written using XUL and JavaScript. So you can put the chrome URL in to the address bar and open various tools/ester eggs. Today CC.Org’s going to list the coolest Firefox Chrome URLs. Feel Free to try them if you are using Firefox. Sorry IE fans! you’ll have to find Firefox for this.

chrome://browser/content/browser.xul
Opens another Firefox, inside a tab in the the existing Firefox window.
chrome://browser/content/preferences/preferences.xul
Opens the Options dialog box inside the Firefox tab.
chrome://browser/content/bookmarks/bookmarksPanel.xul
Opens the “Book Marks Manager” inside a tab in the Firefox window
chrome://browser/content/history/history-panel.xul
Opens the History Panel in the Firefox tab
chrome://mozapps/content/extensions/extensions.xul?
type=extensions
Opens the Extensions window in the current tab.
chrome://browser/content/preferences/cookies.xul
Opens the “cookies window” inside a tab in the Firefox window.
chrome://browser/content/preferences/sanitize.xul
Opens the “Clear Private Data” window inside the current tab.
chrome://browser/content/aboutDialog.xul
Opens the “About Firefox” Dialog box inside the Ff tab.
chrome://browser/content/credits.xhtml
A scrolling list of names.