Install this add-on: https://addons.mozilla.org/en-US/firefox/addon/padlock-icon/

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The padlock only shows that there is a secure connection and doesn't guarantee that you are connected to the right server. So you might still be connected to the wrong server if you make a typo in the URL and someone has claimed that mistyped URL. The functionality of the padlock has been replaced by the Site Identity Button on the left end of the location (address) bar.

https://support.mozilla.com/en-US/kb/Site+Identity+Button

http://www.mozilla.com/en-US/firefox/security/identity/

This has actually existed since Firefox 3.0