Noticed this little bug a couple times but haven't been able to get it to happen again. Sometimes, when watching a video on my 1440p monitor, it will appear very tiny, super-wide, and have black bars on both sides of the video. See below:
After moving the tab on to my second monitor, which is 1080p, it reverts to normal and stays as such when moving it back on to the main 1440p monitor. I'm currently using Firefox 70.0.1.