Not well at all. I have a new 5K iMac with a 395X and Firewatch really struggles to hit 1080/60. If you turn a bunch of things off you can get 60fps some of the time. At 1440p (which is really a far more attractive resolution on that screen) it's 25fps-60fps, but mostly around 35fps.
That said, it runs terribly on Windows too by all accounts for a lot of people, and worse on the PS4 where it can't even hold 30fps.
There is a metal version of Firewatch incoming, which hopefully will help.