The principle here is that cards can't gain or lose modifiers after they're played. If you block with Firefoxes and *then* get to the point where Humble works, Firefoxes has already been prevented, and won't retroactively be un-prevented. However, if you had gotten down before blocking with Firefoxes, they would be CBP and still work.
You are correct that Humble is effectively CBN, but it also says "while opponent has more cards in hand". If playing SoG puts you even, you aren't CBP, but once you get down again you are.
