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
Display information about Firefox.
Display a verse from the fictional Book of Mozilla
Display some funny facts about Robots
Display home page with a Google search bar.
Display the addons page
about:sync-tabsDisplay tabs synced from other computers (tab syncing has to be enabled).
Display credits in alphabetical order
Display Mozilla Public License
Display licensing information and your rights.
Display the logo used on the about: screen.
Display installed, currently enabled plug-ins.
Display the arguments and options used to compile the Firefox build in use.
Display information about the data in the browser cache.
Display details of crashes submitted by Mozilla's crash reporter.
Display memory usage by Firefox
about:config (use with caution)
Display the Firefox configuration page with a lot of configuration options.
Display troubleshooting information.
Switch to private browsing mode
Display an interface for viewing about last session.
Display the malware protection page used when the browser identifies a page as not safe for viewing.
Display the error page used when an SSL/TLS certificate is untrusted or otherwise invalid.
Display the page used when clicking the rss feed icon in the address bar.
Display the error page used when the browser could not access the requested path.
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.
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