The toolbar has completely been replaced by a new one where the game characters represent the buttons: Luigi, go back; Mario, go forward; Goomba, reload; or the Piranha Plant, access the main page.
The theme reaches further than the main page of Firefox. The option window also makes a little reference to the game. You only have to see the icons that have been placed on the buttons.
Other parts of the browser that don't escape either are the scrolling bar (the typical green pipework you can see in Mario Bros), the favorites menu (where Luigi is equal to the folder item) or the error page (shows part of the main screen of the game).
Don't expect a mushroom to appear from the box with the question mark that has replaced the button to close the tabs. That's the only thing it does: it closes the tab.