Mobile FIRST is a powerful extension for simulating smartphones and tablets directly on your computer, allowing users to check the responsiveness of websites across a wide range of devices. It offers features like video capture, customizable views, and supports capturing high-quality screenshots. With regular updates, it currently supports 58 device models—including smartphones, tablets, and even smartwatches—making it one of the top tools for developers looking to ensure their sites are optimized for mobile users.