hard to tell without picture...possibly a dark raccoon? I do not know where you are for southern Ontario but we do not have Fishers down this far ,very rare, even Algonquin area is really too far south. Fishers are also BIG, like Raccoon size... I might of thought a male mink as they get big but I have never heard or saw a mink ever climb a tree ( weasels don't climb trees either and are small, you would probably think it was a rat first)
sorry I would rule out Fisher ..unless you live near Northbay