My site used to be http without SSL. I recently installed SSL to https. I went into the admin settings and changed the setting from http to https. When my site opens in Firefox on a PC and a Mac it opens as http. When I open it on a Mac in Safari, it opens as https.

What can I do to set some consistency here?

The https setting of Zenphoto is for the url generation but the server must be setup as well. I think you might need to look into your server's settings regarding the domains and https.