If the PIC file that you flash is from modified firmware I heard that you will get an error and firmware version "PIC fw version=0xff" and also you will get that issue if you didn't set the voltage properly.

So possible that the PIC chip is bricked. Do you have an extra PIC chip? Use the extra PIC chip and don't use hot air to solder it use a soldering iron and check if it is in the right position and then repeat the procedure from thierry4wd's post but this time make sure it was from the original stock firmware don't use non-stock firmware this might be the reason why you get the "PIC fw version=0xff".