What I did with Gizmo (and my dog) for curbing the "taking your fingers along with the treats" problem worked pretty well. You'll probably be bit more than a couple times doing it this way, but it'll save your fingers in the long run.
What you do is take a strong-smelling treat, and hold it with your fingers curled around it so that they can smell it, but not grab it. Hold your hand out towards the fox. Generally they'll lunge fangs-first; quickly pull your hand away and say "No! Be nice." Keep your voice calm, but firm. Offer your hand again; if they lunge or snap again, do the same thing as before. Rinse repeat.
Foxes are very fast, so you'll probably get your knuckles bit more than once while doing this--but no matter what, do NOT drop the treat; otherwise you'll just be reinforcing the biting behavior.
Eventually, when you hold your hand out, she won't lunge. She may sniff at it curiously, or she may just ignore it, or she may give you a baffled look. So long as it's not a lunge or a snap, immediately release the treat and praise her gently while she eats it.
Get another treat and offer it in the same way. If she lunges, pull your hand away and say "No! Be nice." again in the same firm tone. Basically repeat what you did above.
Continue until you have given her three treats, then take a break from it by playing a game she likes.
If you do this 3-to-5 times a day, and give her 3-to-5 treats a "session", and keep this up for a week straight, by the end of the week, she shouldn't be snapping at treats.
Once you start this training, though, it's very important that from then on out, you never, ever give her a treat if she snaps at you. "Only polite foxes get treats" should be a rule that is set in stone. If she bites you and gets the treat anyway later on, you'll be right back to square one again.