Fiorina is in the Neroid system, situated within the Outer Veil, according to the Alien<sup>3</sup> novelisation. 

> Fiorina wasn’t an impressive world, and in appearance even less inviting, but it was **the only one in the Neroid Sector with an active beacon.** The EEV’s data banks locked in on the steady signal. Twice the damaged navigation system lost the beam, but continued on the prescribed course anyway. Twice the signal was recovered. Information on Fiorina was scarce and dated, as befitted its isolation and peculiar status.

> ?Fiorina ‘Fury’ 361,?the readout stated. ?**Outer veil mineral ore refinery**. Maximum security work-correctional facility.?The words meant nothing to the ship’s computer. They would have meant much to its passengers, but they were not in position or condition to read anything. ?Additional information requested??the computer flashed plaintively.

Neither of those places exist in the real world, but we can assume that it's somewhere between the supposed location of Zeta2 Reticuli (the location of LV-426) and Earth.