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Because both are trying to be response to the death of Browser Company's Arc. (https://arc.net)

The browser designs look identical to Arc, yes, but the website of these two new “Arc responses” also look the same, down to the background color.

The only difference is zen is Firefox based while arc and nook are chromium based.

According to their FAQ, Nook is WebKit-based.

Oops I misunderstood, I thought the fact that it ran chrome extensions meant it was chromium based. Thanks.