I'd love to but it not being available on Linux and invite-only both makes it very difficult to even give it a try :)

FWIW, I just tried Arc on Windows, but it seems like it doesn't do tree-style tabs at all, unless I'm missing something obvious?

They don't have sync for most things.

Firefox is nice in that not only it has sync of basically everything, but you can run your own server for it.

Arc being such a generic term it'd be helpful to include a link or explanation what that is.

Why? It’s based on chromium anyways. Might as well use Vivaldi instead.