Mar 14, 2018 Jack Wallen walks you through the process of locking down options with Firefox's about:config page to help secure the browser from user tampering. Mozilla / Firefox Help - Editing and Adding Preferences via 'about:config' The Mozilla Suite and Firefox allow you to manually add and edit a large number of preferences (sometimes referred to as 'hiden preferences') with 'about:config' This How-To will show you how this is done.
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 OptionsPreferencesSettingsPreferences 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
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.
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).
Firefox About Config Show Modified
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.
Firefox About Config Android
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.
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. 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).
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)