When a web site supports HTTPS, it works fine. In case of no HTTPS browser hangs. It would be good to display a message like "This web site seems not to support HTTPS" and give a possibility to add this web site to the exception list.

For HTTP, if URL without "www." doesn't work, FireFox (depending on settings) prepends "www." and tries again. When "HTTPS Everywhere" is active, "www." will not be prepended. It is not clear, if this is a FireFox bug or a bug in "HTTPS Everywhere".