Catalyzes the formation of 4-(hydroxymethyl)-2-furancarboxaldehyde phosphate (4-HFC-P) from two molecules of glyceraldehyde-3-P (GA-3-P). 2 D-glyceraldehyde 3-phosphate = 4-(hydroxymethyl)-2-furancarboxaldehyde phosphate + 2 H2O + phosphate kcat is 0.026 sec(-1). Optimum pH is 7.0. Cofactor biosynthesis; methanofuran biosynthesis. Homohexamer. Trimer of dimers. Belongs to the MfnB family.