The whole status bar was redesigned, so now you use the area to the left of the address (green, blue, or plain) to check whether the site is secure.

Site Identity Button | How to | Firefox Help

Other discussions about this change mention an add-on you can use if you really prefer seeing a lock:

firefox 4 where is the padlock bottom right corner | Firefox Support Forum | Firefox Help

Thank you! Sometimes, it's the most obvious things we don't see! Thanks!

As a person who is color blind I find the "new" method of displaying security almost impossible especially on a small 11 or 13 inch screen. 11% of males are color blind to some degree and green is the most difficult of those colors. Whilst for most red-green is an issue personally I have incredible difficulty with grey and green under different light conditions. The icon as it was as a padlock was intuitive and did not require explanation. Until there is a reversion to the old method of warning about security I will use IE for those sites where I wish to ensure there is adequate secutity. This means that IE will be used for all financial transactions or where I am providing personal information.

@Buffalo12, that's a good point about blue-gray vs. green. Did you see that there is an add-on to display the lock?

You can use this extension to get a padlock on the location bar.

• Padlock: https://addons.mozilla.org/firefox/addon/padlock-icon/