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For those coming here in search of Firefox alternatives, I just want to point out https://zen-browser.app/ that is also a fork of Firefox though perhaps a more customised fork... Check it out if you are in the mood for trying a different layout from the default Firefox experience...

@louischance @davidrevoy Yep, perfectly rational... I too have not (yet) given up on Firefox, I have just started compartmentalizing what I use it for. The Firefox sync allows me to still keep parity between the 2 browsers, though given the direction Mozilla is going I am probably going to delete that account quite soon.
The similarity with Arc is what drew me to the Zen browser initially. I also like the amount of space it provides for the content in it's "compact mode" layout.