Native Hor+ support.
There is no in-game FOV setting but the developers revealed how to edit the FOV through the registry: HKEY_CURRENT_USER\Software\CampoSanto\Firewatch\fovAdjust_h2041137991
Right click on fovAdjust_h2041137991 and choose Modify. Set the base to be Decimal.
Our default FOV is 55. Set the value to be 100 times the amount you want to adjust the FOV by. For example, if you wanted the FOV to be 90 you need to increase the FOV by 35. So the value of fovAdjust_h2041137991 should be 3500 (35 * 100).
If you wanted the FOV to be 45, you would be removing 10, so set the value to be -1000.
This is the VFOV of a natively Hor+ game, so 90 seems a bit extreme.
In 16:9 VFOV 55 means HFOV is roughly 85. To bump up the HFOV to 100 you want VFOV 68, so...
68 - 55 = 13
13 * 100 = 1300
...set the decimal value of fovAdjust_h2041137991 to 1300.