Interesting, I haven't heard of safety third from circus circles, I've always known it as more along the liens of if safety were actually the number one priority, no one would actually do anything because it's too risky.

In terms of cybersecurity, I see it as "security first" culture means people rely on the system to keep them safe. "Safety third" (or security third) emphasizes that everyone should already know they are operating in a risky and dangerous environment and take security as a personal responsibility.

It's just a reminder that no one cares about your life more than you do, so stay vigilant and take personal responsibility.

edit just realized I didn't actually answer your question on the first and second priorities.

I suppose First would be the reason the system exists in the first place (buy something online, for example). Second would be the user experience of doing the thing. Security should help you take calculated risks rather than prevent you from taking any risks at all.