I live in Canada's far north, and, I am a Canadian Ranger, a reserve branch of our military. It is currently -43C as I write this. Most guys up here use the US military "bunny boots," the white ones with the valve on the side used to equalize pressure when at altitude. I don't actually have a pair, but, I'd love to get some. Apparently, they are still warm even when things are wet, like stepping in to overflow on the river, etc.
I do not know what "Mickey Mouse" boots are, but, if they are the same as bunny boots, then they are highly rated up here.