I've decided on a mechanic for firing into and out of fog.

You will tell me which direction you want to fire, N, NE, E...NW

For each direction, the probability of hitting an enemy will be calculated then disadvantage is applied if it manages to match an enemy. That way you can stay in or out of fog but still rain arrows in on them.

Throwing the firebomb will be easier now, you may not need to pop out.