Yes, putting freshly baked cookies in the fridge can cause them to firm up. When cookies are exposed to cooler temperatures, the fats in the cookies, such as butter or oil, will solidify, resulting in a firmer texture. The process is similar to how refrigerating cookie dough before baking can lead to thicker and chewier cookies.
However, it's essential to note that refrigerating cookies can also affect their flavor and texture in other ways. Refrigeration can cause cookies to absorb odors from other foods in the fridge, potentially altering their taste. Additionally, refrigerated cookies might become drier over time if not stored properly.
If you prefer firmer cookies, refrigerating them after baking can be a good option, but ensure you store them in an airtight container to prevent them from drying out and absorbing unwanted odors. For the best taste and texture, it's generally recommended to enjoy freshly baked cookies within a day or two. If you need to store them for more extended periods, consider freezing them instead, as freezing can better preserve their quality.