Catalyzes the reduction of fumarate with reduced coenzyme M (CoM-S-H) and coenzyme B (CoB-S-H). In vitro, is able to reduces fumarate with reduced benzyl viologen, oxidize CoM-S-H and CoB-S-H to CoM-S-S-CoB with methylene blue, and reduce CoM-S-S-CoB with reduced benzyl viologen. The enzyme has specificity for the two thiol compounds as the CoB--CoM heterodisulfide reductase. The enzyme is very sensitive to oxygen. coenzyme B + coenzyme M + fumarate = coenzyme M-coenzyme B heterodisulfide + succinate Binds 1 [2Fe-2S] cluster per subunit. Binds 2 [4Fe-4S] clusters per subunit. Optimum pH is7.0. Optimum temperature is 75 degrees Celsius. Subunit B of the heterodimeric fumarate reductase of methanogenic Archaea, composed of subunits A (TfrA) and B (TfrB).