Mozilla Firefox comes with "Private Browsing" feature which allows you to visit the Internet without saving any information about which websites, etc you visited. It might become very useful when you use a shared computer and don't want others to know which websites you visit.
To start private browsing in Firefox, you can either press "Ctrl+Shift+P" keys together or select "Start Private Browsing" option from orange Firefox button in titlebar.
When you start private browsing, Firefox shows a confirmation prompt message as shown in following screenshot:
You need to click on "Start Private Browsing" button to use this feature. The alert box also contains a checkbox "Do not show this message again". If you accidentally or intentionally enabled this checkbox while clicking on "Start Private Browsing" button, Firefox will never show the alert message again in future.
Firefox doesn't provide any option to re-enable this alert box in its Preferences window. So the question is how to restore this confirmation prompt?
Don't worry! You can restore private browsing confirmation prompt using secret about:config page:
1. Open Firefox and type about:config in the addressbar and press Enter. It'll confirm, click on I'll be careful, I promise! button.
2. Now type private in the Filter box and look for following boolean key:
3. The entry "browser.privatebrowsing.dont_prompt_on_enter" would be set to true. Double-click on it or right-click on it and select "Toggle". It'll set its value to false.
4. That's it. It'll immediately restore private browsing confirmation prompt.
BONUS TIP: As you can see there is another option "browser.privatebrowsing.autostart" given for private browsing mode. Its set to false by default. You can set it to true to always start Mozilla Firefox in private browsing mode. You can also enable the same option from Options -> Privacy tab.