Hitting a mob with a fire aspect weapon sets them on fire. When they attack you they set you on fire too, essentially making the enchantment a negative one.

A recent change made it so that when mobs are on fire and they attack you, they have a chance of setting you on fire too. Obviously, this makes mobs burning in sunlight a lot more difficult. However, it has made the Fire Aspect enchantment into a negative enchantment, as using it on mobs will essentially make you prone to fire from the mobs you hit.

e.g. I hit a zombie with a fire aspect sword. He is on fire. He attacks me. I am now on fire because of me.