Natsumi is a really great UI for Firefox-based browsers (excluding zen). It has a similar UI to Arc and Zen but is far more open and customizable than both Arc and Zen, and is really stable as well. While it does run on top of existing browsers, it adds a lot of actual useful features, and not only what arc provided. I love that it's getting frequent updates and unlike zen it doesn't break every other time it gets updated. It's a really great project and I highly recommend checking it out.

https://github.com/greeeen-dev/natsumi-browser