You are trying to add a Node<T> to a collection that holds "something that is a subclass of Node<T>". What if your fringe happens to be an ArrayList<SubNode<T>>? Clearly, you are not allowed to add some Node<T> when it expects a SubNode<T>. One solution is to remove the upper bound on your fringe: simply make it a Collection<Node<T>>.
I've not mentioned that the code was as you told to be a Collection<Node<T>>. But on design progress I needed a subclass of Node<T> called HeuristicNode<T>. Somewhere else I've gonna need to add that subclass object to fringe. That's why the fringe must be as it is declared.
I think the neatest way is to probably just make it a Collection<Node<T>> and then simply add HeuristicNode<T> to it when you need to. Then when you retrieve nodes, simply check whether the node is a HeuristicNode: