I count that including tonight's ep (where I didn't see the ep in my time zone, and missed the first part where he got fired), he's been fired at least 3 times (4 if you count his layoff at the hands of the Germans).
First firing: He drives around one of those golf cart thingies, and while waving at Bart, he crashes into something and gets canned by Sherri and Terri's father.
Second firing (if you count it): Homer drifts off into a daydream while being interviewed by the Germans where he's in "The land of chocolate", and wakes up ten minutes later. He's the only one who got canned.
Third firing: Homer attempts to enlarge a donut in the nuclear reactor or the core, he causes a fire, and claiming it's his "first day", he gets spared, but when Burns asks why Homer lied to him, he said it was his first day again and gets fired.
Fourth firing: Whatever happened in last night's ep because I missed that part.
I don't count the ep where Marge had Maggie because Homer quit, and he did come "crawling back" when he needed more money.