How to calculate Pareto Optimal outcome in a game with a Nash Equilibrium
For Pareto optimality, you can ignore the timing. Also, anything you know about Nash equilibria in this game is irrelevant.

Let $v_F(w, L)$ be the payoff of the firm and $v_U(w, L)$ be the payoff of the union given $(w, L)$. What you are looking for are pairs $(w, L)$ such that neither the firm nor the union can have a higher payoff without the other side having a lower payoff. Equivalently, you want to the payoff of the firm to be as large as possible under the constraint that the union cannot have a lower payoff and you want the payoff of the union to be as large as possible under the constraint that the firm cannot have a lower payoff. 

This means that $(w,L)$ is Pareto optimal if and only if it solves the following two maximization problems:

$$\max_{(w',L')} v_F~\text{ s.t.}~v_U(w',L')\geq v_U(w,L).$$
$$\max_{(w',L')} v_U~\text{ s.t.}~v_F(w',L')\geq v_F(w,L).$$

Now if you just have to find one Pareto optimum and not all Pareto optima, you can just maximize $v_F+v_U$.