The ornamental design for a concrete firebowl, as shown and described.
FIG. 1 is a top perspective view of the concrete fire bowl embodied in the present invention;
FIG. 2 is a top view of the concrete fire bowl from FIG. 1;
FIG. 3 is a side view of the concrete fire bowl from FIG. 1;
FIG. 4 is a bottom view of the concrete fire bowl from FIG. 1; and,
FIG. 5 is a side perspective view of the concrete fire bowl of FIG. 1.
The dash-dash-dash broken line shown in FIG. 5 is included for the purpose of illustrating environmental context and forms no part of the claimed design.