Interrupt priority order is fixed in hardware.
The fixed polling order is IE0, TF0, IE1, TF1, TIRI on a standard 8051. This is only important if 2 interrupts occur at exactly the same time. If an interrupt occurs a fraction later than another one it has to wait until the first interrupt is finished.

The priority level system allows you to give one or more interrupt sources a higher priority than others. The only difference in handling is that a high priority interrupt can interrupt a running low priority interrupt. But even then a high priority interrupt will have to wait if another high priority interrupt is still running.

Happy programming