The far-infrared spectroscopy of the troposphere (FIRST) instrument is a Fourier-transform spectrometer developed to measure the Earth’s thermal emission spectrum with a particular emphasis on the far-infrared. FIRST has observed the atmosphere from both the ground looking up and from a highaltitude balloon looking down. A recent absolute laboratory calibration of FIRST under ground-like operating conditions showed accuracy to better than 0.3 K at near-ambient temperatures (270–325 K) but reduced accuracy at lower temperatures. This paper presents calibration results for balloon-flight conditions using a cold blackbody to simulate the space view used for on-board calibration. An unusual detector nonlinearity was discovered and corrected, and stray light was measured and removed. Over most of the range of Earth scene temperatures (205–300 K), the accuracy of FIRST is 0.35–0.15 K (one sigma).