Yet Another Chromium Fork. In a way, it's almost a shame Chrome was open sourced through Chromium. Maybe if it weren't, we'd have a healthier, more diverse browser ecosystem.

Writing a browser from scratch is hard.

Hard is underselling it. At this point it’s borderline impossible. It’s unlikely we will ever see a new browser.

Why, licensing/proprietary issues?

Web browsers are the most complex codebases in existence. Firefox contains more lines of code than Linux. It would require probably 100 developers just to keep up with new features being added. Then going back and catching up on the already existing components is virtually impossible.

Mozilla had a project called Servo which aimed at replacing just a small slither of Firefox and the project ran for years with a huge monetary investment and eventually was canceled.

Haven't some parts of it been integrated in Firefox?

The project does still seem to go on, although outside of Mozilla: https://github.com/servo/servo