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 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.

Warning: Changing these advanced settings can sometimes break Firefox or cause strange behavior. You should only do this if you know what you're doing or if you are following trustworthy advice.

Opening about:config

Type about:config in the address bar and press EnterReturn.
A warning page may appear. Click I accept the risk!Accept the Risk and Continue to continue to the about:config page.

Click Show All 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: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.

Note: Preference names are case-sensitive. Search terms are not.

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

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.

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 or to remove an added preference, right-clickcontrol-click on the preference and select Reset. If you added the preference entry yourself using about:config, it will no longer be listed after restarting Firefox.

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.

Note: A Delete

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, right-clickcontrol-click anywhere in the list of preferences. Select New from the context menu, select the type of preference you are adding (String, Integer or Boolean) and follow the prompts to enter the preference name and value.

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

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)

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