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.

Switched to zen recently, and although I only expected a slightly different experience to firefox, it's hugely better. Profiles/containers/workspaces especially are great.. this level or organization fits my mental model much better and and I never need to manage bookmarks or use multiple windows. (Performance with large numbers of tabs seems much better too, presumably inactive workspaces are reclaiming the memory in smart ways).