The headphone jack on the Project is redundant with the main outs. Therefore you'd need to set the main outputs 1/2 as a cue mix.
As found on page 18 of the FSProject Manual:
https://pae-web.presonusmusic.com/downloads/products/pdf/FireStudioProject_OwnersManual_EN.pdf
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The Main Output as a Cue Mix:
It is possible to designate the main output in Audio I/O Setup as a cue mix. This is helpful if you often record yourself and require quick access to zero-latency monitoring for live inputs. When the main output is designated as a cue mix, a Zero Latency button will appear on any audio channel with an assigned audio input in the Console, below the Mute, Solo, Record, and Monitor buttons.
