  The payload of the Faint Intergalactic Redshifted Emission Balloon
(FIREBall-2), the second generation of the FIREBall instrument (PI: C. Martin,
Caltech), has been calibrated and launched from the NASA Columbia Scientific
Balloon Facility (CSBF) in Fort Sumner, NM. FIREBall-2 was launched for the
first time on the 22nd September 2018, and the payload performed the very first
multi-object acquisition from space using a multi-object slit spectrograph
(MOS). This performance-oriented paper presents the calibration and last ground
adjustments of FIREBall-2, the in-flight performance assessed based on the
flight data, and the predicted instrument's ultimate sensitivity. This analysis
predicts that future flights of FIREBall-2 should be able to detect the HI
Ly\alpha resonance line in galaxies at z~0.67, but will find it challenging to
spatially resolve the circumgalactic medium (CGM).
