Here are all of the useful (and not so useful, but still funny) about: triggers you can type into your address bar.
about: — The same page as “Help -> About”.
about:blank — A blank page. Useful for setting as your homepage.
about:crashes — List of Breakpad crash reports, with links and datestamps. Only in products based on Gecko 1.9 or later (Firefox 3, Thunderbird 3, SeaMonkey 2).
about:credits — The list of contributors to the Mozilla projects.
about:plugins — Lists all your plugins as well as other useful information.
List is courtesy of Mozillazine. From the useful to the silly, which of these is your favorite?