With the exclusion of the bottom toolbar in Firefox 4, the security padlock icon no longer shows. To verify if a site is secure, look in the address bar at the top of the page. If the beginning of the website starts with https:// (as opposed to http://) and the website's icon and company name show on a green or blue background (consult the attached image), then the website is secure.

The padlock icon has been removed in Firefox 4.0.
You can click on the Site Identity button to verify if the connection is secure or not. See this article for details: How do I tell if my connection to a website is secure?.

If you need the padlock icon, you can try this add-on: Padlock.