I've started using Orion for its great "Install this site as an app" feature. It's the best way I know of to create a site-specific browser on Mac. I now use Orion for my Gmail (and a few other things).
However, while my default system browser is still Firefox[1], Gmail opens external links in Orion. This makes the site-specific browser much less useful, since Orion isn't logged into my other sites.
To make the site-specific browser really feel like an app and not be a leaky abstraction, it should open links in your default web browser. If your default browser is already Orion, then nothing changes from current behavior, but otherwise it feels passive-aggressive, like "you must use Orion for everything or this feature won't work well for you".
[1] Orion is great, but I'm not making that jump right now. For one reason, certain extensions I depend on (Vimium) still don't always work properly.