Well Firefox have a special URI (Uniform Resource Identifier) scheme to display internal pages, or built-in functions. These commands start with an "about:" followed something, for example - "about:blank" displays a blank page.
Here is a list of rarely known Firefox Hacks
about:
Display information about Firefox.
about:mozilla
Display a verse from the fictional Book of Mozilla
about:robots
Display some funny facts about Robots
about:home
Display home page with a Google search bar.
about:addons
Display the addons page
about:sync-tabs
Display tabs synced from other computers (tab syncing has to be enabled).
about:credits
Display credits in alphabetical order
about:license
Display Mozilla Public License
about:rights
Display licensing information and your rights.
about:logo
Display the logo used on the about: screen.
about:plugins
Display installed, currently enabled plug-ins.
about:buildconfig
Display the arguments and options used to compile the Firefox build in use.
about:cache
Display information about the data in the browser cache.
about:crashes
Display details of crashes submitted by Mozilla's crash reporter.
about:memory
Display memory usage by Firefox
about:config (use with caution)
Display the Firefox configuration page with a lot of configuration options.
about:support
Display troubleshooting information.
about:privatebrowsing
Switch to private browsing mode
about:sessionrestore
Display an interface for viewing about last session.
about:blocked
Display the malware protection page used when the browser identifies a page as not safe for viewing.
about:certerror
Display the error page used when an SSL/TLS certificate is untrusted or otherwise invalid.
about:feeds
Display the page used when clicking the rss feed icon in the address bar.
about:neterror
Display the error page used when the browser could not access the requested path.
about:permissions
Display permissions given by you to different websites, like allowing to store passwords, share your location, allow cookies, open pop-up windows, and maintain offline storage.
Thanks for reading, If you know more, Please let us know through comments.