"but the interrupt priority says that a serial interrupt is number 9 in priority and the the external interrupt 0 is number 3 in priority"

I think you are referring to a thing called "priority within level". This simply means that if several interrupts would be detected at once, which one would get serviced (the others will not be serviced, until the current one is handled).
Your external interrupt actually can get in the way of the serial interrupt as you describe, but only if if:
- your interrupt handlers are much too long (Erik sais use 7 bytes for a handler, personally I think that's a bit harsh and you can afford to use more)
- you misengineered or misused the external interrupt. If you connect something to the external interrupt pin which can flood the controller with external interrupts so it doesn't get to spend much time on anything else anymore, you have made a bad design choice, and you should go back to the drawing board. You should ask yourself for instance in such a situation if it wouldn't be better to poll the external thingie, or use external hardware to handle it's speed.

But by no means can you have a situation where a flood of interrupts can cause the stack to overflow.