If I am signed into a Google account (which we always are, with Gmail) going to a YouTube page will cause the embedded video to not resize appropriately to the size of the window and it will tend to be overlaid over other elements on the page. Attaching a result of going to https://www.youtube.com/watch?v=juJL26dafvs It is important to copy-paste that into the address bar and not just click on a thumbnail from the main youtube page. When resizing the window, the video stays in the same position and keeps the same size. Full screen and theater modes also don't work. This bug does not happen if I am not signed into a Google account. I have no extensions installed. I have tried to refresh Firefox. I have restarted Firefox in safe mode and the bug still occurs.