About: about page in Firefox

What is sometimes observed in the address bar of a blank page “about: blank”, for Mozilla, became the philosophy of access to browser resources. Few people know about this philosophy, except enlightened monks, for whom the address bar is less sacred and more instrumental than for the rest. But it is difficult to remember all the variety of properties of this line. Open the window to enlightenment helps mantra "about: about". Now, at the time of version 37, it opens 34 options for continuations of links after the colon. There are undocumented.

Jokers rub their hands: you can call the most natural puzzling pages with minimal effort. But there are a number of useful pages. The purpose of the list below is to highlight the share of useful pages for an advanced browser user.

There are pages in this list that add various add-ons if they are installed in the browser. For example, about: scriptish, about: stylish-edit, about: treestyletab-group. (Previously, it was about: speeddial, but now for some reason it stopped this practice without changing its useful functions.)

This is even written in another holy book: https://developer.mozilla.org/en-US/docs/about:_protocol or more generally, for Mozilla products - kb.mozillazine.org/About_protocol_links .
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Not all of its pages are equally valuable. We describe the most important of them. (It makes sense to open all of them in browsers on the Mozilla engine - Firefox, Seamonkey, K-Meleon, by copying the fat dough into the address bar.)

about: about - a list of the “officially” valid pages of this pseudo-protocol. Although, for the advanced user, only some of these links are important, listed here.
about: addons - an important page for managing add-ons (for those who use them; there is a call to it from the menu - Tools - Add-ons or Ctrl-Shift-A)
about: cache - current general data about browser caches (how much memory it takes)
about: config - important - browser settings and manual management
about: customizing - call the settings of the browser GUI (as per the View menu - Toolbars - Customize ...)
about: memory - detailed statistics on occupied memory (by processes)
about: newtab - Speeddial in Firefox-ovsky (9 links with partial pages).
about: permissions - Access and rights to previously visited sites and the ability to change them.
about: plugins - plugins (which are not addons)
about: preferences - Surprisingly, these are the settings for Firefox in a slightly different full-page format. Missing only the tab Options --Tabs for some reason.
about: support is a very useful dialog about important browser settings. Here the easiest way to get a list of addons with versions. Although the name is not invoking, therefore the real chance to get to it is to remember the mantra " about: about "
about: downloads - the list of downloads (Ctrl-J (in Win)), but in a tab, not in a special window

Less essential and non-essential windows:

about: - version, browser build
about: accounts - settings synchronization prompt (technical page, usually called from settings)
about: app-manager - apparently, the page of remote control devices under Firefox OS
about: buildconfig - browser build information
about: crashes - information about the dates of the browser crashes (I have 19 for the year, for 2,200 hours of work, usually from plugin-container with many (> 20) open tabs)
about: credits - the names of the project participants
about: healthreport - statistics of the browser operation time and its starts
about: home - the official start page
about: license - legal links to browser software licenses
about: logo - browser logo
about: mozilla - an egg-like text on the theme of Mozilla (slightly more: www.mozilla.org/book , goes back to the opposition of IE-Phoenix)
about: privatebrowsing - warning about opening a private window
about: rights - legal text on user rights
about: robots - Easter egg (from Fx 3.0b5) Read more - bugzilla.mozilla.org/show_bug.cgi?id=417302
about: sessionrestore - the session recovery dialog (empty; for filling the windows with information you need parameters in the URL, which is true for other similar pages)
about: sync-log - settings sync actions
about: sync-progress - the screen saver waiting settings
about: sync-tabs - tabs received during sync
about: telemetry - interesting data on the speed of processes in the browser
about: welcomeback - dialog about restoring browser settings (judging by the text - after canceling addons management)

Undocumented (non-essential or requiring parameters for full usefulness):

about: blocked by default.
about: certerror - Displays This Connection is Untrusted if a web page has invalid certificate
about: feeds - Feed viewer template.
about: neterror - “Oops. Firefox can't load this page for some reason. ”
about: socialerror - TBA "Firefox is unable to connect with (null) right now."
about: tabcrashed - TBA. Firefox 25+. “Tab crashed. Well, this is embarrassing. We tried to display this Web page, but it's not responding. ”

What was it?

Internals of the browser, more or less designed by developers for advanced users, interspersed with links to similar, but garbage pages, useful only for internal use in addons and plugins.

UPD 2015-07-17 : interestingly, another interesting “about” works in Fx, but does not appear in their list: about: reader? Url = here_insert_url. For example, for this article:

about:reader?url=http://habrahabr.ru/post/255345/ 

As you can see, just like this, the “Reader View” icon does not appear in the address bar on this page, and the article's parser works for the specified special link.