if the critter is latched on you only have one arm/hand to use. lots easier to talk about what you'll do than do it.
When did it become impossible to consider a given situation, analyze it from all the angles you can think of, use existing knowledge to fill in gaps and then make a plan?
Foxes, in general, are small, light-boned critters around 2 feet long in the body or so, going < 20 pounds easy.
That makes them similar to a large domestic cat, but without the claws. I have experience controlling those while they are angry and I am lightly dressed.
If a fox is hanging off of your forearm (the most reasonable place for a small-mouthed animal to leap up and grab), you can simply batter it against a tree, rock or the ground until it dies or lets go.
It will be in front of you and thus accessible to your free hand.
If you kneel, it will be possible to get it pinned down with your legs while you do what you have to with your free hand.
If you already know you can draw a weapon (knife/gun/whatever) with both hands, that means you pretty much know you can kill it.
I'd say that, much as I have a pretty good idea what I'd do in most SD situations because I've thought them through and practiced a bit, I now have a reasonable plan on what to do if a fox ever latches on to my arm. I can tweak it as the circumstances change.