If I buff a nit that is already on 5+ might, does it become mighty for the Fiira legend ability?

No, it does not.

According to Rule 709: "A Unit 'becomes Mighty' at the moment its Might changes from being less than 5 to being 5 or greater."

If a unit is already at 5 or greater Might, it is already "Mighty" (Rule 708). Increasing its Might further does not change its status from "not Mighty" to "Mighty," so it does not "become Mighty" and will not trigger Fiora's Legend ability.