A <-> B <-> C <-> D <-> A...

// A is firstNode, D is lastNode

        if ( length == 1 )
        {
            firstNode = null;
            lastNode = null;

            firstNode.next = null;
            firstNode.prev = null;
        }

        else
        {
            Node secondNode = firstNode.next;
            Node secondToLast = lastNode.prev;

            firstNode.next = null;
            firstNode.prev = null;

            lastNode.next = null;
            lastNode.prev = null;

            secondNode.prev = null;
            secondToLast.next = null;

            firstNode = null;
            lastNode = null;
        }

That should send everything in between as candidates for garbage collection, I hope?