App for Messenger is a browser extension designed to provide an easy access to Facebook Messenger from your desktop. This unofficial app allows users to open Messenger in a standalone, resizable window that is separate from the Firefox browser. It operates similarly to Google Chrome Apps, ensuring that once closed, it does not run in the background, thus keeping your system light and responsive. The application requires a Facebook account for usage and emphasizes a simple interface with auto-saved settings for convenience.

The extension features an 'Always-On-Top' mode, allowing the Messenger window to stay above other applications for effective multitasking. It includes a user-friendly toolbar with six buttons that offer quick access to various functions. The toolbar is designed to disappear after a few seconds of inactivity but reappears when the mouse hovers over the right side of the window, making navigation intuitive and seamless.