Mission Planner firmware?

Can someone please provide information which version of firmware does the Mission Planner use. Is it the latest ArduPlane (3.x+) version or some other?
If I want to use Mission Planner with Pixhawk do I have a choice to select ArduPlane 3.1 firmware?

Mission planner does not use firmware at all, the airframe does. You upload new firmwares to the autopilot from Mission Planner, you can select many different versions from a drop down menu.