YouTube Fullscreen Theater Mode is a free Chrome extension designed to enhance your video viewing experience on YouTube by maximizing the theater mode to utilize the full height of the browser viewport. This add-on effectively adjusts the layout for both standard videos and livestreams, ensuring an immersive experience. By scrolling the page, it hides the YouTube header, allowing for an uninterrupted view of the content.

Additionally, the extension resizes the chat feature to fill the entire height of the viewport, making it ideal for users who enjoy interacting during livestreams. Importantly, it does not alter the page when theater mode is disabled, preserving the standard viewing experience when preferred. This tool is a simple yet effective enhancement for YouTube users seeking a better way to enjoy videos.