I just got a new phone (Moto G8 Power) with Android 10 and playing videos has several problems. I have the latest Firefox (68.11.0) installed and I tried the Beta version and another version as well, same problems. When I play Youtube videos, Firefox shifts the video to the right side of the screen going into the "navigation button" area and crops some of the video as well. I uploaded 2 screenshots as you can see how the "good" one is far more centered and that also allows you to compare it to the "bad" and see how much of the video actually gets cropped off. I was able to get Firefox to play the fullscreen video properly using an extension (YouTube High Definition) that allows you to set the video size to "full page" but that only works sometimes so not really a good workaround at all. There's also other sites where videos won't even play like Vimeo, which works fine on my old phone with an updated Firefox installed. I've tried several other browsers (Edge, Chrome, Vivaldi) and they all play YouTube videos properly (not opening in the YouTube app either). I much prefer the browsing of Firefox overall but for videos it's a no-go right now.