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Step 1: Type about:config in the address bar. (Take a look at the screenshot above to get an idea). You will receive a warning: Proceed with Caution as Changing advanced configuration preferences can impact Firefox performance and security. Just click on Accept the Risk and Continue. (In case, you do not want to receive the warning in the future, you can opt-out of the notification by removing the tick mark beside Warn me when I attempt to access these preferences).