If u r a web developer or know HTML somewhat then u might have come across a about:blank address/parameter. Actually the developers need to include some more about: addresses for various reasons. These about: addresses are most of the time browser specific and they refers to internal or say hidden pages.
What follows is a list of such about: pages for mozilla firefox!

about: A simple prompt without any parameter will display the Mozilla about information. Since it’s accessible from the top menus as well, it’s not quite a hidden feature. (This is equivalent to using Help >> About Mozilla Firefox option.)

about:blank A blank page. Useful for setting as your homepage.

about:buildconfig Obviously, it will show the build platform configuration and parameters. Well if u r not a programmer then u may find only junk!

about:cache Displays cache statistics and disk cache directory location. This has following two suboptions.

• about:cache?device=memory Lists memory cache entries.
• about:cache?device=disk Lists disk cache entries.

about:config GUI for modifying user preferences (prefs.js). The most complex and meaningful of them all. Careful on what you plan to change here, your browser won’t run properly after if you play with the wrong things.

about:plugins This option will display detailed information about all the plugins installed for Mozilla Firefox. (Note plugin is different from extension)

about:credits It’s time to give the credit to whoever deserves it. There’s an alphabetically sorted list of all the people that gave their contribution to the development of Firefox.

about:Mozilla This is a weird thing. It will display the so-called Book of Mozilla. Something similar was present in Netscape too. There’s actually no real book although the quotations might give you that impression.

about:license Shows the Mozilla Public License and the Netscape Public License for the piece of software.

Well moreover some extensions add their own about:’-thing like:
about:chrome
about:language
about:tmp
about:flashblock

Wait its not over yet! We love firefox for many reason and one of them is availability of extensions for everything! Well there is a MR Tech About:About extension for this. It adds the following “about:” options to the Help menu:

• about:
• about:config
• about:plugins
• about:about
• about:buildconfig
• about:cache
• about:cache (Disk)
• about:cache (Memory)
• about:credits, about:license
• about:mozilla