Big bugs and big water. This isn't the land of braided currents and dainty rises that most people think of when they hear "Firehole". This is big, bouldery water with vicious slashes at huge flies.
The salmonflies are in — you can see them on the banks, you can turn over a rock and see so many nymphs that they're hard to count. The adults are cruising from tree to tree, grass to rock like apache helicopters — big, slow and shockingly aloft.
The first few days that we saw the adults the fish hadn't keyed in on them yet. They were to busy gorging themselves on nymphs in the shallows. With some stealth and a low-profile you can sometimes watch these big fish, some coming out of the Madison, crusing the soft water near the banks looking for the heavily-armored nymphs to make a break for it.
Once the adults had hit the water for a few days the trout started looking up instead of down. It started with little fish, easy to convice, but hard to hook. Another day or so and the bigger fish lost their caution and started attacking anything salmonfly sized that hit the water.
After a few sore lips these fish start to get wary, they require a better presentation and often whack at a fly before actually taking it. Not setting the hook until the fish actually eats the fly requires practice, and nerves of steel. I usually get good at it right around the time the hatch starts peetering out.
The bugs are here, we're in the thick of it. They don't stick around long, so if you want to hit this one, now's the time.