A firm will only employ additional workers to what point?

When the marginal gains from hiring that employee are less than the cost of the employee. I.e. if the employee costs $40,000, but the firm only nets $30,000, that employee will not be hired.

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Theoretically, when the value produced by that worker, or their return on investment isn't high enough... but that gets mixed up with a lot of political stuff plus a desire to make money without regard to improvement of the product, so sometimes the resultant value isn't even factored in. Spending is just frozen unless it inhibits the decision-maker's personal goals.