Also worth mentioning is the other (than Vivaldi) let's recreate classic Opera project: http://otter-browser.org/
Of course both of these are very very young projects and aren't really ready to replace Firefox.
I always really wanted a browser that'd be just a bunch of programs adhering to UNIX philosophy and then striving for simplicity. I mean, components interacting together by talking to each other through the simplest means available, with simplest protocols possible, and with every component being freely swappable. But without a DIY approach to things - instead, trying to bring a Firefox/Chrome-like experience by default, but being hackable and with everything very loosely coupled under the hood.
uzbl is somewhat close, but it's not really portable and still a has a pretty big monolithic webkit engine component, that handles rendering, scripting and networking.
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It's probably high time to write a libre browser that focuses on being completely customizable. And not focus primarily on speed. I'm bewildered by those who are deadset on that over all else, it just serves into the goals of for-profits who can dump tremendous resources into optimizing to bamboozle away freedom.
Freedom doesn't cost millions of dollars. A libre Vivaldi, with a tighter feature scope (no email client etc) is where it would be at.