Catalyzes the sequential condensations of isopentenyl pyrophosphate (IPP) with geranyl diphosphate (GPP) to yield (2E,6E)-farnesyl diphosphate (E,E-FPP), with E,E-FPP to yield geranylgeranyl diphosphate (GGPP) and with GGPP to yield nonaprenyl diphosphate (PubMed:23091471, PubMed:30301210, PubMed:32495977). May also have weak activity with dimethylallyl diphosphate (DMAPP) (PubMed:23091471). (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate (2E,6E,10E)-geranylgeranyl diphosphate + 5 isopentenyl diphosphate = all-trans-nonaprenyl diphosphate + 5 diphosphate Binds 2 Mg(2+) ions per subunit. kcat is 0.3 min(-1) with GPP as substrate. kcat is 0.7 min(-1) with FPP as substrate. kcat is 0.17 min(-1) with DMAPP as substrate. Isoprenoid biosynthesis; farnesyl diphosphate biosynthesis; farnesyl diphosphate from geranyl diphosphate and isopentenyl diphosphate. Isoprenoid biosynthesis; geranylgeranyl diphosphate biosynthesis; geranylgeranyl diphosphate from farnesyl diphosphate and isopentenyl diphosphate: step 1/1. Contains the canonical two aspartate-rich DDxxD motifs, designated as FARM (the first aspartate-rich motif) and SARM (the second aspartate-rich motif). The primary role of the FARM and SARM is the chelation of the divalent magnesium ion cofactors that assist substrate binding and catalysis, but it may also play a role in determining product chain length. Belongs to the FPP/GGPP synthase family.