Firefox for Android (beta as well) forces landscape orientation when a video enters fullscreen, even when Android rotation is locked.
Older Firefox versions exposed the preference media.videocontrols.lock-video-orientation, allowing users to prevent that behavior in about:config.
Please restore this preference, or add a visible setting such as: “Do not change screen orientation in fullscreen video.”
This would allow portrait fullscreen video and respect the user’s Android orientation lock.