When obtaining all kinds of credit/loans they look at your employment record to see how long you stay at each job and whether you will remain employed during the repayment period. They have no way of knowing why you left each position so you being fired should not affect your credit history.
however, if you've changed jobs every couple of months and have lapses in your employment history, will reflect poorly on your entitlement to receive a mortgage. Then again, if you're a first time buyer, CHMC will probably insure you anyways.
Good Luck dude!
