Indeed. I now have firefox configured to always try HTTPS first. A warning pops up if it doesn't work, and you have to click through it if you want to proceed. It turns out that a large number of sites not only have http://example.com redirect to https://www.example.com, as you describe, but https://example.com doesn't work at all -- resulting in this warning. Clicking through it, you end up at https://www.example.com.