Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.
Arcane Comet can be unreliable, but if procced from MF's [E] it should be easy to hit. Arcane Comet also scales very well with Bonus AD.
Because MF is a caster AD Carry, the mana sustain here allows you to poke throughout lane with [Q].
Celerity's bonus Movement Speed synergizes well with Miss Fortune's natural Movement Speed from [W]. The secondary effect of Celerity also allows MF to gain a considerable amount of bonus AD when her [W] is active.
Scorch on MF is a great lane harass and early-game rune thanks to her ease of proccing with [E]. The Magic damage also helps diversify MF's damage type a bit in the early game to she is a slightly more difficult to itemize against.
Triumph allows you to take close fights in the early laning phase and turn that single kill into a double thanks to the bonus healing.
In late game teamfights the heal can be enough to allow you to continue channeling your [R] through taking damage to kill the entire enemy team.
WhyCoup De Grace
Coup De Grace synergizes incredibly well with MF's massively high damage [Q] and [R], putting any squishy target with poor positioning in easy kill range.