Hidden pages within Mozilla Firefox

The hidden pages of Firefox are configuration items that allow end users to see and make changes to the way Firefox operates. Let's have a try on it by typing the command in the address field.

1. about: - Shows the about information for the Firefox application. Similar to the usual Help ? About menu option found in most applications (and in Firefox, too).
2. about:buildconfig - Displays platform configuration information used in the running build of Firefox.
3. about:cache - Shows the caching information stored for Firefox.
4. about:plugins - Displays the installed plug-ins running within the current install of Firefox.
5. about:credits - Shows the brains, and the coding brawn behind the Firefox project.
6. about:Mozilla - Displays quotations from the Book of Mozilla.
7. about:config - Modifies configuration parameters used by Firefox
   

Source:Finding the hidden bells and whistles in Firefox by Derek Schauland