"When you visit a Secure website in Firefox chances are
that the FavIcon for the website is replaced with a Green bar with
the details of the company. This is because of the default
properties in Firefox to display detailed information of the
website from the Extended Validation Certificate on the website.
However, if the website doesn't host a Extended Validation
certificate then the website URL (link) in the address bar is not
highlighted or in otherwords shows as a normal website URL.
"However, just to give you that extra highlighting of a website
thats claims to be secure, you can choose to display the website
domain or the subdomain highlighted in a Blue background next to
the FavIcon of the website."