Firefox’s user interface is written using XUL and Javascript. The Gecko engine which renders the webpages also renders the user interface. This means you can enter chrome URLs into the Firefox address bar and do some really cool things.

I wouldn’t recommend trying out any of these "tricks" if you have any important websites open. Everything has been tested with Firefox 1.5 on Windows.

chrome://browser/content/browser.xul

This is awesome. Type this in the address bar and you can have a nested copy of Firefox running inside Firefox. You can even open up several tabs at this URL and have tabbed windows. You can even nest Firefox inside itself several times. Most things seem to work.

chrome://global/content/alerts/alert.xul



Notice: This will make your Firefox window disappear under the taskbar. Before loading this URL, make sure you have at least one other tab open. The Firefox window should do a crazy dance in the corner of the screen. If your Firefox window disappears, right click Firefox in the taskbar and select maximize. Press Control+W to stop Firefox from moving around.

chrome://browser/content/preferences/preferences.xul



Opens the Options window. In Firefox 1.0 use chrome://browser/content/pref/pref.xul

chrome://browser/content/bookmarks/bookmarksPanel.xul



Opens a list of your bookmarks.

chrome://browser/content/history/history-panel.xul



Opens a list of websites in your history.

chrome://global/content/console.xul



Opens the Javascript console.

chrome://mozapps/content/extensions/extensions.xul?type=extensions



Opens the Extensions window.

chrome://mozapps/content/extensions/extensions.xul

Opens the Themes window.

chrome://mozapps/content/profile/profileSelection.xul



Closes the current tab in Firefox; or the window if there is only one tab open.

chrome://global/content/finddialog.xul



Opens the old Firefox "Find" dialog (non functional)

chrome://global/content/printdialog.xul



Opens the Firefox Print dialog (non functional)

chrome://global/content/filepicker.xul



Seems to open a Linux style file picker on Windows for me. Non functional on Windows.

You can use the DOM Inspector and inspect windows to find cool URLs. Let me know if you find something neat.