The Configuration Editor (about:config page) lists Firefox 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 advanced settings that are not present in the OptionsPreferences dialog. This article describes how to use the about:config page to view, change, or reset Firefox preference settings, if you need to do so.
In the address bar, type about:config and press EnterReturn.
To add a new preference, right-clickcontrol-click anywhere in the list of preferences. In the context menu, select New, select the type of preference you are adding (String , Integer , or Boolean) and then follow the prompts to enter the preference name and value.
To change the value of an existing string or integer preference, right-clickcontrol-click on the preference and select Modify from the context menu (or double-click on the preference) and then type in the new value in the window that opens. For a boolean (true-false) preference, right-clickcontrol-click on the preference and select Toggle from the context menu (or double-click on the preference) to toggle its value between true and false.
To reset a preference to its default value or to remove an added preference, right-clickcontrol-click on the preference and select Reset. You can only use the "Reset" option for user set preferences, which are listed in bold type. If you added the preference entry yourself using about:config, it will no longer be listed after restarting Firefox. (To reset all preferences, see the article Reset Firefox preferences to troubleshoot and fix problems).
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)