It has been replaced by the site identity button, for details on using it see the How do I tell if my connection to a website is secure? article.

If you prefer having a padlock, you can use the Padlock add-on - https://addons.mozilla.org/firefox/addon/padlock-icon/

The lock icon has disappeared, but not the security. I imagine the old lock icon was too easy to spoof and so not doing any good.

Rant
(The Powers That Be have also damaged the system for {in some firefox versions} demonstrating phishing and attack protection works! see )

You now click on the space to the left of the location bar and follow the information from there. Whilst using this forum you can try that now, you will then see a padlock.

(Tony beat me to a reply)