I claim the ornamental design for a furniture base, as shown and described.
FIG. 1 is a perspective view of the furniture base; the structure shown in phantom forming no part of the claimed design;
FIG. 2 is a top view of the furniture base; the structure shown in phantom forming no part of the claimed design;
FIG. 3 is a bottom view of the furniture base; the structure shown in phantom forming no part of the claimed design;
FIG. 4 is a left side view of the furniture base, the right side view being a mirror image thereof; and the structure shown in phantom forming no part of the claimed design;
FIG. 5 is a front view of the furniture base; the structure shown in phantom forming no part of the claimed design; and,
FIG. 6 is a back view of the furniture base; the structure shown in phantom forming no part of the claimed design.