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?