Well, the rate-determining-step is attack of the nucleophile to the aromatic ring, which disrupts aromaticity, as opposed to loss of the leaving group.

The rationale as to why F accelerates the reaction relative to Cl and Br is that F is highly electronegative, which creates a large partial positive charge on the carbon it’s bonded to, making it more electrophilic (and increasing the rate of this first step).

You will put a CH3O in place of Br

Start by drawing the product of addition to the aromatic ring at each of the positions where there is a fluorine. Which anion will be more stable?

The best leaving group is F, but other groups can be leaving groups as well like Cl, Br, and OTs.