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@vertigo That seems to be the case wirh Firefox Quantum, indeed. Although not all of Firefox has been re-implemented. I believe I read months ago that only the CSS engine was rewritten in Rust **and** shipped. It's quite possible that other parts of Firefox have been re-implemented since then as well.
In parallel there's also the Servo browser (being written from scratxh in Rust) which isn't yet very usable at this time, but I'm hopeful it can deliver even better results if it does away with the resourcehognness of current browsers.