Trip report
Mar 26, 2018 — Starting from the Mouse's Tank Trailhead, follow the super-popular trail through Petroglyph Canyon to the unremarkable Mouse's Tank, which is just a series of keeper potholes. Bypass the Mouse's Tank drop-off just to its left and then continue down the wash. You'll pass through short section of narrows, where there's a short Class 3/4 drop (sometimes there is a stack of rocks to help get up/down). Just past here is another drop-off, avoidable on the left via a Class 2 slope. The rest of the hike down Fire Canyon Wash follows a sandy wash with gorgeous and chaotic red sandstone features all around you and no more scrambling. I went the entirety of the wash over to the eastern end of Valley of Fire State Park, but you can turn around at any point.
Unfortunately, Ephemeral Arch apparently broke a few years back, but you can still see Thing Arch, as well as a million other little natural arches everywhere. I noticed one particularly large arch on the cliffside, and as a side-trip, find a Class 2+ route up to that arch to get a nice overhead view of Fire Canyon Wash. I didn't include this side-trip in the hike stats (see map for where it's located).