Mozilla Firefox offers its users a wealth of customizations. Just about every component and feature of Firefox can be manipulated. Although the most common Options are available from the Tools menu, advanced users may want further control. That is where the Preferences come in. This little known tool essentially brings the Firefox user behind the scenes to access the root commands. However, manipulating Preferences is not recommended for the novice user, since misconfiguration can cause errors in your Firefox browser.
Click your Firefox icon.
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Type "about:config" in the address bar, and press "Enter."
Click "I'll be careful, I promise!" in the warning window, which states changing Preferences can be harmful Firefox.
Double-click any "Boolean" type preference to toggle between True and False to enable or disable the preference. Double-clicking "Integer" or "String" types opens a dialog box where you can change the value.
Right-click anywhere in the list, and select "New." Choose "String," "Integer" or "Boolean" to create a new preference, and fill in the appropriate details.