Catalyzes the sequential condensation of isopentenyl pyrophosphate (IPP) with the allylic pyrophosphates, dimethylallyl pyrophosphate (DMAPP), and then with the resultant geranylpyrophosphate (GPP) to the ultimate product farnesyl pyrophosphate (FPP). Has a 4.5 time greater affinity for GPP versus DMAPP. (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate dimethylallyl diphosphate + isopentenyl diphosphate = (2E)-geranyl diphosphate + diphosphate Binds Mg(2+) or Mn(2+). Maximal rates of activity with 1 to 4 mM Mg(2+) and only one third of activity with Mn(2+). No activity with Zn(2+) or Ca(2+). Optimum pH is 6.5. Optimum temperature is 40 degrees Celsius. Isoprenoid biosynthesis; farnesyl diphosphate biosynthesis; farnesyl diphosphate from geranyl diphosphate and isopentenyl diphosphate: step 1/1. Isoprenoid biosynthesis; geranyl diphosphate biosynthesis; geranyl diphosphate from dimethylallyl diphosphate and isopentenyl diphosphate: step 1/1. Highly expressed in shoots. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the FPP/GGPP synthase family.