The ornamental design for firearm buttstock, as shown and described.
FIG. 1 is a perspective view of the firearm buttstock with various ancillary firearm components in phantom lines and not a claimed part of this invention;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a side view thereof;
FIG. 6 is an opposite side view thereof;
FIG. 7 is a front view thereof;
FIG. 8 is an end view thereof;
FIG. 9 is a perspective view with an ancillary shoulder portion shown in phantom lines and not a claimed part of this invention in an extended condition; and,