Sometime back, Stylo a.k.a Quantam CSS is available for testing in Firefox Nightly. Till now to test it, user needs to flip the about:config pref ‘layout.css.servo.enabled’, which no longer required for some, as latest update for Firefox Nightly has enabled it by default.
If you’re one of them, to confirm this, visit about: support page, you’ll notice ‘true (enabled by default)’ line for Stylo under ‘Application Basics’ section. Because of this change, you’ll notice Firefox loading stuff much faster on many sites.
Recently, Nightly has entered version 57 and got a new icon. Till now of Quantum project, Quantum Compositor already made available with Firefox 53 ( which speeds up Firefox and makes the browser more stable by preventing crashes on Windows), now, Quantum CSS a.k.a Stylo has been landed in Nightly and enabled by default for some users.