LOL yeah, I don't know if they'll steal or not. TBH I'm mostly concerned for chickens and rabbits, as a rabbit we were trying to befriend was killed by a fox on my lot on my first day of play. ☹️ Cats are vaguely rabbit-sized but I think that if foxes harmed cats someone would have said something about it before now, also cats are more heavily armed. 😉
All that said... it just seems like very un-wild-animal-like behavior for a wild fox who hasn't been befriended by my Sims to run into their house and dash around playfully. I think a non-befriended wild fox would be afraid to do that. I mean, maybe I shouldn't be complaining about un-wild-animal-like behavior in a pack that lets me pet wild rabbits and sing to wild birds and accept gifts from my wild animal friends, lol, but for whatever reason wild foxes who haven't been befriended running into my Sim's house is the immersion-breaking thing? I guess suspension of disbelief only goes so far!