The Firth River flows approximately 150 miles from the Brooks Range through Ivvavik National Park in Canada's northern Yukon Territory, ultimately emptying into the Beaufort Sea. It is considered geologically Canada's oldest river and passes through four distinct sections: the Aufeis Reach, the Mountain Reach, the Canyon Reach, and the Delta Reach. The river is rated Class III–IV and is best suited to rafts, making it a technically demanding trip appropriate for experienced paddlers.

The standard put-in is at Margaret Lake (68.6366°N, 141.5872°W), where floatplanes can land on the lake itself and wheeled aircraft can use a strip across the river. The standard take-out is Nunaluk Spit (69.5076°N, 139.4856°W) on the Beaufort Sea coast, where a Twin Otter can land. Some groups continue paddling to Herschel Island (69.5537°N, 139.2240°W), though pack ice can make this option impossible, and it is generally advised to take out at the spit. Access to and from the river requires charter flights; Inuvik, Northwest Territories, is the nearest jet-served airport.

Paddling the Firth requires a backcountry permit through Ivvavik National Park, at a cost of $9 CAD per person per day. Private party access is extremely limited — only 20 days per year are allocated to private groups, meaning roughly 40 private visitors annually. A separate Parks Canada fishing permit is required for Arctic Char fishing, which is described as excellent. The park has no roads, facilities, established trails, or campgrounds. A Parks Canada station at Sheep Creek is only periodically occupied during summer months.

The canyon section contains the most significant whitewater, with Class III and IV rapids. The upper headwaters near the Alaska border are small, braided, and typically covered in aufeis (river ice) until August. The river's waters are described as clear and cold. The surrounding landscape is above treeline, with the British Mountains providing alpine terrain accessible for hiking.

Wildlife along the river includes the Porcupine caribou herd, numbering approximately 250,000 animals, which ford the river during their annual migration. Muskox are also regularly encountered. The Engistiak archaeological site along the river contains Inuvialuit artifacts and tools dating back over 8,000 years, as well as Pleistocene megafauna fossil remains. Commercial guided trips typically run in late June and July.