Fir Mountain stands in Ulster County, New York, within the Catskill Mountains and the Big Indian Wilderness of Catskill State Park. Located between Big Indian Mountain and Spruce Mountain, it is significant for how its slopes divide water between two major Atlantic watersheds. Its east side drains into Esopus Creek, which flows to the Hudson River and New York Bay. Its west and south sides drain through Biscuit Brook to the West Branch of the Neversink, then the Delaware River and Delaware Bay, while its northwest end drains into Elk Bushkill before reaching Esopus Creek.