I am having the same problems in trying to understand what this means and how to rid myself of it. I am using Firefox also.
From what I gather, does the Accepted Solution mean that the cause of this warning is merely junk on the webpage I am looking at that causes the warning?
Is the webpage really and truly secure except for the commercial junk? I ask because the webpage I want is an Order Form where I will have to send my sensitive information over in order to order a product I want. I must know if the webpage is secure or not.