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). Only geranyl diphosphate (GPP) can be used as isoprenyl acceptor. (2E)-geranyl diphosphate + isopentenyl diphosphate = (2Z,6E)-farnesyl diphosphate + diphosphate Binds 2 magnesium ions per subunit. kcat is 229 sec(-1) with geranyl diphosphate as substrate and 282 sec(-1) with isopentenyl diphosphate as substrate. Homodimer. Belongs to the UPP synthase family. Z-FPP synthase subfamily.