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 the full list of Firefox 4’s about: pages.
1.about: Entering about: in Firefox 4 address bar shows Firefox logo, version, links to pages having list of contributors, licensing information and build configuration etc..
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.