I don't think there's any hard and fast rule on this subject (as with all wildlife, they don't seem to read the text books lol).
I personally prefer very dark nights with a breeze but not too much wind, but as long as it's not foggy, and you can see to spot the foxes, you'll find and get them during most conditions, though there's many other factors to take into account, as the other two lads have said. IMO it's more important getting to know the land and the habits of the foxes that are there, finding the favoured runs and hunting spots etc. I feel that there's no substitute for getting out,