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 The functionality of the padlock has been replaced by the Site Identity Button on the left end of the location (address) bar.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

If you still want a padlock icon also then

https://addons.mozilla.org/en-US/firefox/addon/padlock-icon/

Thank you James. I wasn't even aware that it had gone until I asked my bank why it was taking me so long from Password to actually logging in to my account and were my details safe because of this delay. Their answer was that I was 'safe' as long as the padlock icon was showing. So they were not at all helpful. But you have answered my question fully and even given me more information. Thanks again.