We've confirmed the latest updates to High Sierra are causing graphical issues with integrated GPUs (like the Intel Iris chip).
We have a temporary fix for those playing on Steam only. Once we have a more thoroughly tested and solid patch, we will update the game on all platforms. We appreciate your patience!
On Steam: There is now a public beta branch of Firewatch named "macbeta." To enable this: right click on the game, go to Properties. In the betas tab, switch the drop down from "None" to "macbeta." There will be an ~1.2 GB patch applied that uses Apple's metal renderer.
You may see graphical issues such as full screen errors on startup or shimmering pixels on the ground occasionally (again, we're continuing to tinker and test to find the best solution possible) but the game should be fully playable!