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This is the reason you are getting a crossed lock on firefox. It happened to me a couple of weeks ago.
The solution is really simple: if you have the cetificate for www.yoursite.com or yoursite.com stick to the one you have the certificate for.
That means that if you are certified for yoursite.com you can't have any links or images that come or go to www.yoursite.com because these are insecured items.
The moment you remove all insecure items, the moment you will stop receiving this kind of messages on IE and you will get the normal lock on Firefox.