If you believe Firefox "it’s fast. Really fast. It’s over twice as fast as Firefox from 6 months ago".
It will be as fast as any browser soon.
But the main feature is fix for Meltdown (CVE-2017-5754) and Spectre (CVE-2017-5753, CVE-2017-5715):
See in deed. This post is written with Firefox 57.0.4...