The drop down list for pick previous firmware doesn’t have ArduPlane 3.9.(3-10) available. I also cannot upload a firmware by selecting a custom firmware.
Cube Black. 2 Windows 10 computers and 1 Windows 7. MP versions 1.3.66 and 1.3.68.
It seems Michael is behind the schedule of refreshing the historic fw database.
Till he have time to catch up, you can always download previous versions from http://firmware.ardupilot.org/Plane/ and upload as custom firmware.