about:config is a feature of Mozilla applications which lists application settings (known as preferences) that are read from the profile files prefs.js and user.js, and from application defaults. Many of these preferences are not present in the Options or Preferences dialog. Using about:config is one of several methods of modifying preferences and adding other "hidden" ones.
Warning: Modifying preferences can, in rare circumstances, break Firefox, Thunderbird or the Mozilla Suite, or can cause strange behavior. Only do so if you know what you are doing or are following trustworthy advice.
- In Firefox and Mozilla Suite/SeaMonkey, type about:config in the Location Bar (address bar) and press Enter to display the list of preferences, as shown below in Firefox 3. (If you see a page with the warning message, This might void your warranty!, click the button labeled "I'll be careful, I promise!", to continue.)
- In Thunderbird 1.5 or later, about:config is accessed via "Tools -> Options -> Advanced -> General -> Config Editor (button)". For earlier Thunderbird versions, use the AboutConfig extension.
Alternatively, choose Tools – Options – General and specify about:config as the Thunderbird Start Page. Then ensure View – Layout – Message Pane is checked and choose: Go – Mail Start Page
- For Nvu, there's a compatible version of the AboutConfig extension here.
Adding modifying and resetting preferences
To add a new preference, context-click (right-click) anywhere in the list of preferences. In the context menu, select New then select the type of preference you are adding.
- String is any sequence of text
- Integer is a number
- Boolean is true-false
To modify an existing preference, context-click (right-click) on the preference, select Modify and type in the new value. In some cases, Toggle will be the selection offered for boolean (true-false) preferences.
To reset a preference to its default value or to remove an added preference, context-click (right-click) on the preference and select Reset. If you added the entry via about:config, the preference will no longer be listed after restarting the program. (For more information about resetting preferences, see this article).
You can use the Filter bar at the top of the about:config page to filter the preferences that you want to inspect. Starting with Firefox 3, the Filter bar also accepts wildcards and a regular expression in the form of
/regexp/i to filter preferences.  The i switch is for ignore case. That is the default for normal searches, but you need to specify it if you want the RegExp match to be case insensitive. The RegExp tests for a match of name;value of a preference.
security*ssl security*ssl*rsa security*tls /^security[.]ssl3.*rsa|security[.].*tls/i /^security[.]ssl3.*(128|256)/i /^(?!gecko[.]handler|network[.]IDN|network[.]protocol|print|security[.]ssl)/i /^(?=browser[.]|dom[.]|network[.](?!IDN|protocol))/i
- The article about:config entries contains a list of valid about:config entries and their possible values. The article is a reference for what each preference does; it is not the place to actually edit your preferences! (See the section Opening about:config above.)
- Editing configuration
- About protocol links — other about: pages.
- A Brief Guide to Mozilla Preferences - MDC
- How To Modify Hidden Preferences Using about:config — MozillaZine's step by step guide with screenshots (from web.archive.org)
- Mozilla Help: Editing Configuration files — (from ilias.ca)
- Preferential extension — An extension similar to about:config with extra features, such as documentation for some preferences.