The Firefox Configuration Editor (about:config page) lists application settings known as preferences that are read from the prefs.js and user.js files in the Firefox profile and from application defaults. Many of these preferences are not present in the OptionsPreferences dialog.

Opening about:config

Type about:config in the address bar and press EnterReturn.
A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.

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, and then select the type of preference you are adding.

• String is any sequence of text
• Integer is a number
• Boolean is true or false

To modify an existing preference, context-click (right-click) on the preference, select Modify and type in the new value. For boolean (true-false) preferences, Toggle will be the selection offered.

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 using about:config, the preference will no longer be listed after restarting Firefox. (To reset all preferences, see the article Reset Firefox preferences to troubleshoot and fix problems).

Searching for preferences

You can use the Search: box at the top of the about:config page to quickly find specific preferences. For example, if you enter snippet in the Search box, it will bring up the browser.aboutHomeSnippets.updateUrl preference. Note that preference names themselves are case-sensitive but search terms are not.

Based on information from About:config (mozillaZine KB)