Based on the provided reviews, there is no direct mention of the C Firma™ Day Serum helping to strengthen the skin barrier. The reviews primarily focus on its brightening effects, hydration, and overall skin improvement. However, some users did mention that it provided a firming effect and improved the texture of their skin, which could indirectly suggest some benefits to the skin barrier. For a more definitive answer, it would be beneficial to look into the product's ingredients and their known effects on the skin barrier.