Much like to Google Chrome’s about: pages Firefox 4 has also got some hidden about: pages that shows information about memory usage,cached files,plug-ins,add-ons and much more.Some Firefox 4’s about: pages might confuse you,some pages are for diagnostic purposes only and some might be omitted because they require query strings, here is full list of Firefox 4’s about: pages.
2.about:add-ons : Entering about:add-ons will open Add-ons Manager.
6.about:crashes : This page will show crash reports submitted by Mozilla crash reporter to Mozilla.
19.about:sync-log, about:sync-log1,about:sync-tabs pages shows information related to Firefox Sync that synchronized your tabs, and other info.
Finally you can get list of all these about: pages in Firefox 4 in a single about page that is with about: about.