Catalyzes the condensation of only one isopentenyl pyrophosphate (IPP) unit in the cis configuration to E-geranyl diphosphate (E-GPP) generating the 15 carbon product (2Z,6E)-farnesyl diphosphate (Z-FPP or EZ-FPP). Z-FPP is the precursor of decaprenyl diphosphate, which has a central role in the biosynthesis of the mycobacterial cell wall. (2E)-geranyl diphosphate + isopentenyl diphosphate = (2Z,6E)-farnesyl diphosphate + diphosphate Binds 2 magnesium ions per subunit. Inhibited by citronellyl diphosphate. Optimum pH is between 7 and 8. Homodimer. Belongs to the UPP synthase family. Z-FPP synthase subfamily.