Firing order is 1-5-3-6-2-4-1-5-3-6-2-4-1-5-3-6-2-4-1-5-3-6-2-4-1-5-3-6-2-4. It does that about 600 time per minute at idle.
Seriously, Are you talking about cross-fire from the wires?.
I also tought about that possibility when I had the old sparkplugs wires. (Same backfires then). Same thing with the new wires.
It only does it when the engine is at normal runnig temperature and has been sitting off for 5 to 15 minutes.
jackd