FullScreenTheaterModeforYT by BrijeshMalaviya

FullScreen Theater Mode makes YouTube's theater mode truly fill your browser. Press T on any video and the player expands edge-to-edge — full viewport height with no side gaps or rounded corners — while the video's aspect ratio is always preserved with automatic black bars, so 16:9, 4:3, ultrawide and vertical videos all look correct at any window size.

The title, comments and suggestions move below the fold; just scroll down to reach them. Normal view, true fullscreen (F), Shorts and embedded players are never touched.

Includes a toolbar popup to enable/disable the enlargement and to optionally auto-switch every video into theater mode. No accounts, no tracking, no network requests — your two toggle settings stay in the browser's local storage. Works in Firefox and Zen Browser.