what concerns do you have regarding firefox’s security?
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?
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.