I've run MacBooks for the last 10 years now (early 2006 MacBook, late 2012 MacBook Pro which I specced up via the Apple Store website, and a 2011 MacBook Air which I ran alongside both).
Never used an antivirus or firewall on any of them.
Never had a problem as a result.
My original (06) one finally gave up the ghost in 2013 when it wouldn't hold any charge and kept destroying rather expensive chargers.
Even at 7 years old it ran quicker than a brand new Windows laptop I got from work. 7 years unprotected and it had slowed down ever so slightly due to the amount of crap I'd put on it, but never caught a virus in that time.
Everyone else I know with Macs (which is increasing numbers every year, probably 60% of people I know now) don't bother with anything either.