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Zen Browser - A very nice, Firefox-based, clean UI browser
(zen-browser.app)
This is a most excellent place for technology news and articles.
Can you use FF extensions with it? The web site doesn't say one way or the other.
Definitely as already answered, but I'll also add they have a "theme store" which is kind of a bespoke userChrome.css tool. It isn't just themes, but it is in addition to, not in lieu of Firefox extensions.