If you use Firefox Focus for its privacy features, you'd love its latest version for iOS. You can now set it up so that it auto-locks when it's relegated to the background. The only way to unlock it if you choose to switch the feature on is by verifying your identity with the biometrics saved on your iPhone. After you download the new version, you'll find a section under Settings that says "Use your Touch/Face ID to unlock app." That way, whatever you browse before you navigated to another app would remain private even if you lend your phone to a friend.