From the settings menu, tap Security, then Lock Screen Password to make it so that your Kindle Fire requires a password each time you turn it on. (blog.the-ebook-reader.com)
Enable this along with option which says «verify lock screen password before unpinning» (techpp.com)
It comes with some new features like Ultimate Power Saver Mode which allows you to do only the basic tasks like calling, texting amongst other things and majorly improves smartphone's battery life; and Secure Zone which allows you to configure apps, accounts, wallpapers, and lock screen passwords in different time zones to save you from eyes constantly peeking in your phone. (techpp.com)