The padlock has been replaced by the site identity button, for details on using it see https://support.mozilla.com/kb/Site+Identity+Button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

One of the main principles of Human Computer Interaction [HCI] is that there should be immediate visibility to the system's status. Forcing the user to "click" the identity site key and try to locate the lock-pad encryption icon is poor design. The icon has become a default standard expectation of the INTERNET user community across browsers. Internet Explorer, of which, I'm not a fan - has done away with a bottom bar, yet retained the padlock in the top right of the menu bar [it is conditionally displayed only when the site is secure]. Firefox has made a fundamental UI mistake, and amazingly trumped by the generally inferior Microsoft team. Don't follow the FF team and parrot the propaganda - 'yea we screwed up but you can determine the same thing by taking several extra steps- because we are FF team and you are just the peon users". Death of the open source community?

@Fred

Did you even bother to read the support article about the Site Identity Button?

It's either Green or Blue for web sites that have valid identity information. That alone is more information that a "lock" provides in other browsers, plus if you do click that button you can see far more information that a lock ever provided.

Firefox has used th4e Site Identity Button since Firefox 3.0 was released in June 2008, it's not new in Firefox 4. What's new with Firefox 4 is that the "lock" is finally gone. How long should a transition period last? Isn't 2 1/2 years enough for users to learn something new?

If you think the lock icon is important, feel free to install that extension.