Cross-firing is a way to create “virtual” amounts of money (in other words, the money doesn’t really exist) by depositing checks back and forth between two banks.
Perhaps you use two different banking institutions and find yourself a little short in order to pay your bills this month. Rather than taking out an outrageously expensive payday loan, you decide to try a little cross-firing scam. So you write a check from Bank 1 (where you have no money) and deposit it in Bank 2 to pay your bills. Since it might take a couple of days for the check to clear from your Bank 1 account, you have time to write a check from Bank 2 (where you also have no money) and deposit it back into Bank 1.
This could go back and forth for days, until you can come up with “real” money. Banks are on the look-out for this kind of scam, so don’t try it at home (or anywhere else). Large companies also use this technique, where they send checks back and forth between divisions, in order to make it appear they are more profitable than they really are.
Hope you look good in stripes. This practice is absolutely illegal. The "scam" part of the term should have given it away.