We report on a femtosecond ∼8.5 μm, ∼2 μJ source based on the intrapulse difference-frequency generation (DFG) of 2.1 μm pulses in an AgGaSe2 (AGSe) crystal. Compared to the conventional ∼0.8 or 1 μm near-infrared (IR) pulses, a ∼2 μm driver for intrapulse DFG can provide more efficient conversion into the wavelengths longer than 5 μm due to a lower quantum defect and is more suitable for the non-oxide nonlinear crystals that have a relatively low bandgap energy. Using 26 fs, 2.1 μm pulses for type-II intrapulse DFG, we have generated intrinsically carrier-envelope phase-stable idler pulses with a conversion efficiency of 0.8%, which covers the wavelength range of 7-11 μm. Our simulation study shows that the blueshift of intrapulse DFG is assisted by self-phase modulation of the driving pulses in AGSe. The idler pulses are particularly useful for strong-field experiments in nanostructures, as well as for seeding parametric amplifiers in the long-wavelength IR.