From MozillaZine Knowledge Base
There are a few special URLs, which begin with about:, that you can type into the Location Bar. (In Thunderbird, which has no Location Bar, or in SeaMonkey, you can also use them as "Mail Start Page".)
Listed about: URLs
about: URLs that are listed in about:about page.
- about: — The same page as "Help -> About" (except that, on some applications, Help → About is a much smaller popup).
- about:about — Lists all these about: URLs (Mozilla Suite/SeaMonkey or Firefox 4 and newer only).
- about:accounts — Firefox Sync account. New on Firefox 29. 
- about:addons — The "new" addons manager, in trunk builds dated 2010-05-11 or later.
- about:app-manager — new on Firefox 26. App Manager, a tool to help users build and install web apps on compatible devices (i.e. Firefox OS).
- about:buildconfig — Reveals details about your Mozilla build options.
- about:cache — Displays cache statistics and disk cache directory location.
- about:cache?device=memory — Lists memory cache entries.
- about:cache?device=disk — Lists disk cache entries.
- about:cache-entry — Shows information about a cache entry. Used in about:cache links. Requires parameters.
- about:config — GUI for modifying user preferences. (Also accessible in Thunderbird by means of the "Config Editor" button in the "Advanced" Preferences).
- about:crashes — List of Breakpad crash reports, with links and datestamps. Only in products based on Gecko 1.9 or later (Firefox 3, Thunderbird 3, SeaMonkey 2).
- about:customizing — Only available on Australis.
- about:credits — The list of contributors to the Mozilla projects.
- about:data — opens the Data Manager which shows per-site data and permissions. This is part of SeaMonkey 2.x and available as a Firefox extension.
- about:downloads — new on Firefox 20. (Also in recent versions of SeaMonkey. Jökulsárlón Download Manager may be required.)
- about:home — Firefox Start Page
- about:healthreport — new on Firefox 21 (bug 718066, bug 719484). Also accessible via "Help -> Firefox Health Report".
- about:licence and about:license — both show the Mozilla Public License and the Netscape Public License for the piece of software. ( Only in products based on Gecko 1.8 or later)
- about:logo — Displays the application logo (Mozilla Suite, SeaMonkey, and Firefox 3).
- about:memory shows the memory usage of Firefox starting in Firefox 3.6 (Windows) and Firefox 4 (Linux). Implemented in bug 515354
- about:mozilla — The famous Book of Mozilla
- about:networking — new on Firefox 26.
- about:newtab — the default new tab page since Firefox 13.
- about:permissions shows and manages the permissions as also shown in Tools → Page Info → Permissions. Implemented starting with Firefox 6 (bug 573176)
- about:plugins — Lists all your plugins as well as other useful information.
- about:preferences — Firefox only. Open preferences.
- about:robots — Easter egg, see bug 417302. Beginning with March 8th, 2008 trunk builds, it's now available on Firefox 3.0b5.
- about:support — Starting in Firefox 3.6 (bug 367596) and SeaMonkey 2.1a trunk builds based on Gecko 1.9.3a (bug 518607). Displays information such as Application Basics (version, link to open the profile folder, link to about:plugins and about:buildconfig), Extensions, Modified Preferences and, starting in Firefox 4, Graphics information (bug 587102). Also accessible via "Help -> Troubleshooting information".
- about:telemetry — new on Firefox/Thunderbird 19 and SeaMonkey 2.16 (bug 661881)
- about:welcomeback — new on Firefox 25.
Unlisted about: URLs
about: URLs that are not listed in about:about page but exist.
- about:blank — A blank page. Useful for setting as your homepage.
- about:blocked — Used by Safe Browsing tool to display bad web page.
- about:bloat — Displays BloatView output (disabled in release builds).
- about:certerror — Displays This Connection is Untrusted if a web page has invalid certificate.
- about:kitchensink — Was never included in Mozilla for the reasons listed in bug 122411 ("Mozilla does not have a kitchen sink"). You can install a Firefox extension that adds it though :) (In SeaMonkey, you would have to bookmark the above-mentioned XML page, copy it, or add SeaMonkey to the extension's install.rdf. In SeaMonkey or Thunderbird, you could also change the Mail Start Page.)
- about:feeds — Feed viewer template.
- about:newaddon — Dialog shown when one or more add-ons have been added without using the normal Add-on Manager procedures, by dropping it in one of the "special" add-on directories. It asks which ones you want to keep enabled.
- Typing about:newaddon in the URL bar won't display anything. (It may briefly flash an empty error dialog, then you get the empty about:blank page.)
- about:neterror — Displays error page.
- about:socialerror — TBA
- about:tabcrashed — TBA. Firefox 25 and newer.
- about:webrtc — TBA
Certain extensions add a few additional about: URLs.