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. [https://support.mozilla.com/en-US/kb/Site%20Identity%20Button 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: [http://support.mozilla.com/en-US/questions/796225 firefox 4 where is the padlock bottom right corner | Firefox Support Forum | Firefox Help]

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Thanks for the fast response, I can be a bit dizzy sometimes