I'm using Firefox Nightly 130.0a1 on Samsung Galaxy S23 FE. When I open a video in fullscreen and it rotates to landscape, there is a large gap between the video and the edge of the screen. Interestingly, this gap is affected by the setting "Customize -> Address bar location" - If the address bar is set to appear on top the gap is on top, and if the address bar is set to appear on bottom the gap is on the bottom. I've included screenshots from both settings. This happens on both YouTube and Dailymotion, so it is a browser issue and not a site issue.