A nifty little feature of Mozilla-based browsers. It was included in Mozilla 1.3 Beta (and other browsers based on it), Firebird 0.6, and Netscape 7.1.

Typing "about:config" into the address bar brings up a chart of the browser's options. Most of the more useful options there are accessible through the Tools > Options... window, but there are a few useful otherwise-inaccessible options you can tweak to customize your browser even more.

You can modify each option displayed there by right-clicking on it and selecting "Modify". You can also add new options by right-clicking, selecting "New", and then selecting the type (boolean, string or integer) of value you'd like to add. Most changes take effect immediately, but some require a program restart. (Actually, the about:config screen has been in the browsers for a while; the ability to edit the options there is a relatively recent addition.)

A neat part of the display is the Filter. If you enter any text in this box at the top of the page, only the Preference Names which contain the string you enter will be displayed.

Some useful options, in no real order:

Prevent links from creating new windows (disable target="_blank"):
set browser.block.target_new_window to "true"

Turn off <BLINK>:
set browser.blink_allowed to "false"

Change the URL that the throbber points to:
set browser.throbber.url to whatever your heart desires

Search E2 instead of Google IFL: (If you type a non-URL string into the address bar and press Enter, Firefox will do an I'm Feeling Lucky Google search for the string you enter. You can change this behavior to, say, search Everything2...)
set keyword.URL to "http://www.everything2.com/index.pl?node=" and set browser.fixup.alternate.enabled to "false"

Disable automatic image resizing:
set browser.enable_automatic_image_resizing to "false"

Change the default options of View Source:
set view_source.wrap_long_lines to "true" in order to make the View Source window word-wrapped
set view_source.syntax_highlight to "false" in order to disable colored syntax highlighting

set browser.frames.enabled to "false" in order to disable frames completely
set layout.frames.force_resizability to "true" in order to always be able to resize any frames

Prevent middle click from pasting clipboard contents in address bar:
set middlemouse.contentLoadURL to "false"

Change highlighting behavior:
set layout.word_select.eat_space_to_next_word and layout.word_select.stop_at_punctuation to "true"

Make Firefox display pages quicker:
set nglayout.initialpaint.delay to a lower number, but not all the way to 0 (it's rumored to cause instability; mine's at 75)

Disable disk caching:
set browser.cache.disk.enable to "false"

Enable pipelining:
set network.http.pipelining and network.http.proxy.pipelining to "true", and network.http.pipelining.maxrequests to "8" (tweak reports say to set it to 100, but the hardcoded limit is 8)

(If you don't see any of the preferences I mention in your list of options, just right-click and add a new entry with the missing name. And if you mess something up and want the default value back, right-click on an option and select "Reset".)

The display is actually just an easy way of looking at the prefs.js file. This means that it will not override any user-set privileges, as user.js takes precedence over prefs.js. However, this means you can easily break something that will render your browser unusable without further config-file tweaking, so don't go editing entries willy-nilly until you think you know what you're doing. You've been warned.

MozillaZine's page "How to Modify Hidden Preferences Using about:config" (http://www.mozillazine.org/misc/about:config/) has a brief description of the screen (not much more than I've given you, but it's got pictures). More about Mozilla preferences can be read in "A Brief Guide to Mozilla Preferences", at http://www.mozilla.org/catalog/end-user/customizing/briefprefs.html. For the user who wants more options-related toys, check out the Preferential extension at http://preferential.mozdev.org/. Finally, in case you mess something up, there's a list of the default values for each preference (with explanations) at http://preferential.mozdev.org/preferences.html.

The only thing I find lacking in this little feature is a way to read a sentence or two about what a particular option does. Many of the options have pretty descriptive names (like browser.display.background_color), but for the few that aren't intuitively obvious, a built-in way of reading a bit about them would be handy. However, as Firefox is still pre-1.0, I wouldn't be surprised to see it included in a later release. In the meantime, however, a Google search usually answers my questions.