Font Inspector is a Firefox add-on designed for web developers and designers, enabling users to inspect fonts used on any webpage effortlessly. By clicking the toolbar icon, users can access an intuitive in-page user interface that reveals detailed information about the selected font, such as size, style, color, and family. This functionality is essential for assessing typography choices within web design projects.

Additionally, Font Inspector provides important details about the current tab, including its ID, width, height, title, and URL. While the in-page UI is active, other links and functions on the page are temporarily disabled to allow for a focused inspection experience. Users can easily close the UI by clicking the toolbar icon again or pressing the escape key, restoring full page functionality.