Catalyzes the condensation of isopentenyl pyrophosphate (IPP) with geranyl diphosphate (GPP) to yield (2E,6E)-farnesyl diphosphate (E,E-FPP). May be used for squalene and possibly sterol biosynthesis. (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Mn(2+) and Fe(2+) ions also support activity. Dithiothreitol increases the enzyme activity by 2.5-fold. Optimum pH is 8. Isoprenoid biosynthesis; farnesyl diphosphate biosynthesis; farnesyl diphosphate from geranyl diphosphate and isopentenyl diphosphate. This enzyme is unique in that it is the first reported eubacterial E,E-FPP synthase that does not have four amino acids between the aspartate residues of the first aspartate-rich motif (FARM) and that has the features of the archaeal chain length-determining (CLD) region. Belongs to the FPP/GGPP synthase family.