The ornamental design for a firearm accessory mount adapter, as shown and described.
FIG. 1 is a top perspective view of a firearm accessory mount adapter, showing my new design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is front side elevation view thereof.
The rear side elevation view of the firearm accessory mount adapter shown in FIG. 1 is the same as the front side elevation view as shown in FIG. 6.
The broken lines represent unclaimed portions of the invention and therefore form no part of the claimed design. The dash-dot lines are for illustrating boundaries of the claimed portions of the invention and form no part of the claimed design.