Url Security is a useful extension for Firefox that shows you the security level of a web site you are visiting. It turns the URL bar into green on secure sites and yellow on partially encrypted sites.
Of course Firefox has security indicators (it displays https in the address bar and a lock icon in status bar), but not big enough to grab a user attention.
Url Security is designed to solve this problem. As said above, it turn the URL bar green on secure sites (SSL encrypted) and yellow on partially encrypted sites.