There is a lot of useful information about Firefox hidden away behind the about: URL protocol. The most useful is the URL about:config, which displays preferences and settings that can be inspected and changed.

Available about: URLs depend on your specific Firefox version. Here is a complete list of URLs in the about: pseudo protocol:

about: page Description
about:about Provides an overview of all about: pages available for your current Firefox version
about:addons Add-ons Manager
about:buildconfig Displays the configuration and platform used to build Firefox
about:cache Displays information about the memory, disk, and appcache
about:checkerboard Switches to the checkerboarding measurement page, which allows to detect checkerboarding issues
about:config Provides a way to inspect and change Firefox preferences and settings
about:compat Lists overriding site compatability fixes, linked to specific bug issues.
about:crashes Lists all crashes, which happened during the runtime of Firefox (in case the user enabled the crash reporter)
about:credits Lists all contributors to the Firefox project
about:debugging Switches to the Developer Tools debugging page, which allows you to debug add-ons, tabs and Service Workers
about:devtools Summarizes the developer tools and provides links to documentation for each tool
about:downloads Displays all downloads done within Firefox
about:home Start page of Firefox when opening a new window
about:license Displays licensing information
about:logo Firefox logo
about:memory Provides a way to display memory usage, save it as report and run the GC and CC
about:mozilla Special page showing a message from "The Book of Mozilla"
about:networking Displays networking information
about:newtab Start page when opening a new tab
about:performance Displays memory and performance information about Firefox subprocesses/add-ons/tabs
about:plugins Displays information about installed plugins
about:policies Lists out the Firefox for Enterprise policies
about:preferences Firefox settings (also available through Firefox menu > Options)
about:privatebrowsing Start page when opening a private window
about:profiles Display and manage Firefox profiles
about:protections Privacy Protections report consisting of Enhanced Tracking Protection, Firefox Monitor and Firefox Lockwise data
about:restartrequired A page users are sent to when Firefox requires a restart due to an update
about:reader Indicates a web page has Firefox Reader View turned on. See Firefox Reader View for clutter-free web pages
about:rights Displays rights information
about:robots Special page showing notes about robots
about:serviceworkers Displays currently running Service Workers
about:studies Lists the Shield Studies that are installed
about:sessionrestore Session restoration (displayed after a Firefox crash)
about:support Troubleshooting information (also available through Firefox menu > ? (question mark) > Troubleshooting Information)
about:sync-log Displays a synchronization protocol related to the Sync feature
about:telemetry Displays telemetry data collected and sent to Mozilla while Firefox is running (in case the user enabled telemetry)
about:url-classifier Displays the status of the URL Classifier services that Firefox uses (for example for Safe Browsing)
about:webrtc Information about WebRTC usage
about:welcome Page first displayed when Firefox is installed
about:welcomeback Information page displayed after Firefox is reset

These URLs are defined in docshell/base/nsAboutRedirector.cpp within the kRedirMap array. The array maps most of the URLs, like config to URLs in the chrome: pseudo protocol, such as chrome://global/content/config.xul. The about location information is duplicated in docshell/build/nsDocShellModule.cpp.