As I understand it, a cross through the icon means that the site is secure but it draws some content from another server that isn't. This might be an image for example. Firefox picks up on this but presumably, IE ignores it. It seems this would be down to the site rather than Firefox. I don't think this necessarily means there is a security issue. In any event, the bank presents the site to you as a secure and Firefox is also reckoned to be a secure browser. I've not heard of any instances where Firefox has been unsafe to use.