The genre is called a "walking simulator" (not the best name but the one that stuck).
Try: Return of the Obra Dinn, Gone Home, Dear Esther, The Stanley Parable, SOMA (with peaceful mode enabled). I've heard good things about What Remains of Edith Finch, That Dragon Cancer and Everybody's Gone to the Rapture but have not played them.
Spiritfarer is pretty wonderful. You play a psychopomp, helping spirits in the afterlife to move on. But in the process, you ferry them around in a boat that you slowly expand, with all sorts of crafting mini games. The stories of the spirits you meet are mostly melancholy, but very moving, as they try to make sense of their lives and mistakes. I loved it.
Hmm... it depends on what exactly you liked about Firewatch. I personally classify Firewatch as a game with great production quality and no pressure so I can take my time. It's the kind of game I can relax/unwind to. The titles below were exceptional in this regard - you can't go wrong with any of them IMO.
Haven't tried Spiritfarer, but the others on that list don't seem similar.