>When I cleared mine just now, I got a security warning from FireFox when clicking on the Save button (see attached image). This implies that if I was updating the information with a real credit card number, the transmission would not be encrypted, even though the page is nominally SSL-protected.

When you access that page, is it under https. When the post is made, it is also under https. I just tested again and this works. You might want to check for security issue in your browser or else which interferes with that.