Specifically forms (2Z,6Z)-farnesyl diphosphate (Z,Z-FPP) from dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP). Also able to use nerylpyrophosphate (NPP) as substrate. Involved in the biosynthesis of several sesquiterpenes. dimethylallyl diphosphate + 2 isopentenyl diphosphate = (2Z,6Z)-farnesyl diphosphate + 2 diphosphate isopentenyl diphosphate acts as activator of its own catalysis. Belongs to the UPP synthetase family.