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.
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.
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.
Opens the Options window. In Firefox 1.0 use chrome://browser/content/pref/pref.xul
Opens a list of your bookmarks.
Opens a list of websites in your history.
Opens the Extensions window.
Opens the Themes window.
Closes the current tab in Firefox; or the window if there is only one tab open.
Opens the old Firefox "Find" dialog (non functional)
Opens the Firefox Print dialog (non functional)
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.