Generates Z-farnesyl diphosphate (Z-FPP) from isopentenyl pyrophosphate (IPP). Z-FPP is the precursor of decaprenyl diphosphate, which has a central role in the biosynthesis of the mycobacterial cell wall (By similarity). (2E)-geranyl diphosphate + isopentenyl diphosphate = (2Z,6E)-farnesyl diphosphate + diphosphate Binds 2 magnesium ions per subunit. Phospholipid metabolism; decaprenyl phosphate biosynthesis. Belongs to the UPP synthase family. Z-FPP synthase subfamily.