What negative Gate to Source threshold voltage means in MOSFET datasheet?
They're PMOS. Vgs sign is opposite to NMOS.
For a PMOS to conduct, Vgs has to be negative... Simple as that...


  I am looking for MOSFETs that will switch off when Li-ion cell voltage reaches 3.3 V or lower.


You can get integrated LiIon protection chips to do that...