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 OptionsPreferencesSettings panel. This article describes how to use the about:config page to view, change, or reset Firefox preference settings, if you need to do so.
Table of Contents
Opening about:config
Type about:config in the address bar and press EnterReturn.
A warning page may appear. Click to go to the about:config page.
Click to view all preferences currently set in the profile or use the Search preference name box to filter the list.
Searching for preferences
You can use the Search preference name box at the top of the about:config page to quickly find specific preferences. For example, if you enter ocsp in the Search box, it will bring up the security.OCSP.enabled preference.If you enter a search term and a preference does not exist, you will have the option to add a new preference (see below).
Modifying preferences
You can double-click on a preference to change its value or you can do the following:
For a boolean (true-false) preference, click the Toggle 
button.
For a string (text) or a number preference, click the Edit button and enter a new value.
Click on the checkmark to save the change.
Resetting or deleting preferences
To reset a preference to its default value, click the Reset button. To remove an added preference, click the Delete button. If you added the preference entry yourself, it will no longer be listed after restarting Firefox. button may be shown for some preferences that appear in bold type that you did not modify or add, such as preferences whose values are system-specific. Those preferences will be added back, as appropriate, if deleted.You can only reset or remove modified preferences, which are listed in bold type. (To reset all preferences, see the article Reset Firefox preferences to troubleshoot and fix problems).
Adding preferences
To add a new preference, enter its name into the Search preference name box at the top of the about:config page. At the bottom of the search results (if any), there will be the option to create a new preference. To do so, specify the type of value that preference will store:
- String is any sequence of text
- Number is an integer
- Boolean is true or false
Click the Add button to create the new preference and set its value. Boolean preferences are set to true by default; use the Toggle button to change the value. For String or Number preferences, click the Save button. Click the Delete button to discard the new preference.
Based on information from About:config (mozillaZine KB)
