It’s a pain to make sure that every site you enter credentials or nonpublic information (such as credit card info) into is encrypted. I’d be willing to bet that even security aware people get complacent sometimes and assume that since amazon.com was encrypted the last time I used it, it’s probably still encrypted.
Now there’s an add-on for Firefox that makes it much more obvious when the site you’re looking at is or isn’t encrypted. It’s called “SSLPersonas”. Despite its name, you’ll need to look for it under add-ons/extensions, not themes/personas. It dynamically changes the entire web browser theme depending on what kind of page you’re on. By default, it uses your current theme for unencrypted websites (i.e. HTTP). It switches to a blue theme with a lock for HTTPS sites with a traditional certificate (i.e. not extended validation (EV)). It switches to a green theme with locks and a certificate for HTTPS sites with an EV certificate. And, finally, it switches to an orange them with a triangle and exclamation mark for HTTPS sites that have a broken/expired certificate. The themes it uses are customizable, so you could make the broken certificate use a McDonald’s Hamburgalar theme if you wanted.