Numerous posts from the developers on Steam indicate that they do not force a minimum set of specifications - if you do not have their recommended minimum the game will still run. It may not run well but they will not lock you out either.
It is an OpenGL game built on the Unity engine. If you have any other games like that already then you have an idea how things will go (or not go).
Just FYI - I have a 2015 MacBook Air (2.2 Ghz i7 + 8 GB RAM). It's got the Intel HD 6000 card. I installed FireWatch both in Mac OS X 10.11 and in Bootcamp (Windows 10). It will run in both, but the frame rate is significantly better in Windows. It's passable there, but I doubt I'm getting more than 5-7 fps in OS X. The app crashed in OS X until I restarted my Mac. It might have been because I had a 1920x1080 monitor hooked up originally, but even after I disconnected it, the game crashed. A restart solved it.
So, I've got the most powerful MacBook Air you can have, and it's only passable if you want to run it in Windows.
Bought it anyway - turned off all the visual settings and put it down to 720ish.
Wasn't all bad truth be told - not a great frame rate but I bought for the story and can enjoy the visuals on a replay.
One run through so far - wonder if tactical saves can help see different dialogue.
Well my Mac Pro ran it fine as soon as I turned off autosaving, it only chugged when hitting loading areas. Shame I couldn't get it to run higher than 1080p; it just started up as a black screen when I tried to run it on my 4K monitor.