The padlock has been replaced with the site identity button, for details on using it see https://support.mozilla.com/kb/Site+Identity+Button

Ideally you should focus on the new identity button that is explained in the previous post.

However, there is an addon that adds the padlock icon to Firefox 4. https://addons.mozilla.org/en-US/firefox/addon/padlock-icon/

Because the status bar at the bottom of the window is no longer shown by default, this addon displays the padlock inside the identity button. For users who are trained to look for a padlock, this might help them to move their attention to the new identity button.