It's just that if security is a core priority, you'd expect to see this mentioned at least once. Based on what I read, the core priorities are speed, privacy, and convenience.

Compare this to Chrome, Safari, and Edge (tagline: "A fast and secure browser that's designed for Windows 10"): https://www.google.com/chrome/browser/features.html, https://www.apple.com/safari/, https://www.microsoft.com/en-us/windows/microsoft-edge.

Perhaps this is just a marketing or internal communication issue. Did somebody from security engineering have a seat at the table when the Firefox 57 marketing material was discussed?

For what it's worth, in most companies, marketing material is typically created by marketing people, with very limited interaction with engineering.

The message for Firefox 57 is performance. This doesn't mean that other stuff wasn't improved (case in point, better sandboxing), just that you can only tell one story at a time.