macOS-native vertical tabs were a big win for me too, and like yourself, its WebExtensions API is a win over Safari OOTB but there are Chrome extensions I really rely on that don't work, some as a result of the differences between Webkit and Blink themselves, so I find myself drawn to Arc browser where I get, IMO, a much more robust UI where vertical tabs are the default and the command palette in place of an address/search bar is on the money.

Orion is about to release in-place window switching that sorta matches up with Arc's behavior, but I still prefer Arc as the command palette UX, as well as being able to use back/forward mouse buttons to switch 'in-place' windows, is just the superior affordance.

To be fair, Safari also has MacOS-native vertical tabs, in tab groups, that sync group state across the full ecosystem, and do the "in window switching" when you change groups.

// I pay for Kagi, and use Orion for certain extensions.

Yes, it is fairly straightforward and easy enough to use with the Tab Group switcher in the menu, but sadly it does not allow you to nest tabs in a tree manner (probably it was decided it was a power user feature).

What is this "in-place window switching" feature you mentioned?

Its an enhancement on Firefox's original tab groups UI where you don't need to spawn windows to navigate to and act on groups of tabs

Same here. Super happy with Arc. It's very well designed. It solves a lot of annoyances I had with profile management in other browsers

I wanted to try Arc, but somehow can’t get past the login screen. Support from them has not been very responsive yet.

I wish more browsers adopted the approach of Tree Style Tab, the Firefox extension.

(Heck, I wish Firefox natively implemented it.)

Firefox also needs a toggle to hide that unnecessarily large sidebar header. It’s possible to do that with custom userChrome, but there’s no reason it couldn’t be a checkbox somewhere.