Beside some more internal stats and features, it also reveals an Easter egg :)
Here are some of the magic: about: * functions:
about:memory shows how much memory the browser--and any other Web browser--is using. Conveniently for Web developers, it also shows how much each Web site in a browser tab is using.
about:network tracks the detailed network activity of using a Web site.
about:histograms graphs various performance measurements such as the time taken to autocomplete text users type into the browser.
about:crash crashes the active browser tab.