Zen Browser: Firefox-based, open source browser that imitates Arc
It's basically striving to become Arc, but with Firefox as a base.
If you don't know the Arc browser: it's Chromium based (boo) and pioneered some interesting UI components like vertical tab bars as sidebars, multiple spaces, auto-closing un-pinned tabs to clean up for you, split view of multiple tabs, "Little Arc" to open external links in an auxiliary browser window that doesn't clutter your main workspace, and probably other things I never used.
Firefox plug-ins couldn't replicate that many changes to the browser's UI; I looked. But replacing how the Firefox browser window is composed and drawn in the application itself works very well.
The current version is 1.7.x, and 1.8 is on the horizon. It's considered to be both somewhat stable and reliable, and still in flux development wise. (There are no shortcuts to switch spaces a.t.m. for example.) So if you don't know Arc, you don't know what goodness is still in development; if you know Arc, you know how close this is already.
I'm a big Firefox fan (even though I'm less of a fan of what Mozilla is doing nowadays), so this is all happy news and I like everything about this.
Hence the recommendation 
Author at Zettelkasten.de • https://christiantietze.de/
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Comments
Thanks for the notification about this browser. Looking at the selected features on its home page, I agree that most of the features address my needs, but I long ago figured out solutions to these needs in Firefox for macOS.
For example, my version of "Workspaces" is to put related tabs in separate windows and use macOS window management to switch among them; and my version of "Split Views" is to put the pages in separate windows and use a third-party window management app such as Rectangle to quickly arrange the windows on the screen (an extra advantage of this latter technique is that the same keyboard shortcuts work in any app).
Of the 5 features on the home page, the only one I don't already have in Firefox is "Zen Glance". But a new Firefox-based browser is a good thing.