Looks like Arc, would love to migrate out of it after migration, but always worry about maintenance. Creating a browser is "easy", keeping it up to date is a lot of work, and many open-source browsers look semi-abandoned to me.
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plus, you get synchronization across desktop (Zen) and mobile (Firefox for iPhone/Android). since Google limited theirs only to official Chrome, this feature is basically exclusive to Firefox and forks, Arc <-> Arc Search, and Chrome for desktop <-> Chrome for mobile.