Mozilla's engineers have reached a milestone in the development of the next, completely updated Firefox browser, dubbed Firefox Quantum.

Available in Developer Edition, the new browser is the developing results of Project Quantum, Mozilla's attempt to refactor, redesign, replace, and modernize the very core of Firefox.

Developer Edition now includes "Quantum CSS," an entirely new CSS engine written in Rust and based on the Servo parallel browser engine project. Additionally, it fixes 369 performance bugs in Firefox, with a special focus on responsiveness and UI interactions. Lastly, the developers have been overhauling how Firefox prioritizes work, responding more quickly to events like user input while delaying less urgent computations until the browser is idle.

Compared to Firefox six months ago, today's Developer Edition is twice as fast on benchmarks like Speedometer 2.0 that simulate the real-world performance of modern web applications.

Furthermore, Firefox is 64-bit and multi-process by default, and Firefox's architecture allows it to take advantage of multi-core processors while still respecting the available RAM. Meanwhile,it promises significantly reduced crashes caused by buggy graphics drivers.

The developers have also rebuilt Firefox's interface ("Photon") to be faster and more modern.

Highlights include redesigned menus, square tabs, and a new "Library" button that acts as a single place for your bookmarks, downloads, history, etc. By default, Photon combines the search and URL bars into a single widget, but the old style is only a preference away.

The New Tab Page features highlights from your recent history and bookmarks, as well as recommendations from Pocket. Each of these content blocks are optional, and add-ons can completely replace the new tab page to create entirely different experiences.

Mozilla's team also refreshed form handling in Firefox, adding a brand new autofill feature and implementing built-in widgets for <input type=date> and <input type=time> elements.

Lastly, Firefox's preferences were completely redesigned and are now searchable.

Firefox Quantum: Developer Edition also includes a ton of refined, redesigned, and brand new developer tools.

A few highlights:

• The Console, Debugger, and Network tabs are now implemented using standard web technologies, including React and Redux.
• The Inspector gained tons of new features for working with CSS Grid, CSS Variables, toggling classes on elements, etc.
• The Console now supports grouping messages and expanding / inspecting objects in-line.
• The Debugger offers completely new ways to search, navigate, and debug projects.

Future releases of Firefox will include Quantum Render, a new, GPU-optimized rendering pipeline based on Servo's WebRender project, and Quantum DOM Scheduler, a new technique that ensures that tabs in the background can't slow down your active tabs.

You can try out Developer Edition today, or sign up to get notified when Firefox Quantum is released to mainline Firefox.