full screen mode

Is there any setting option for video playback in mozilla firefox app? I try to find it but couldn't. I have problems playing YouTube video in firefox app in full screen. Whenever i tap on the icon for full screen the video goes to full screen but also crops most of the upper and lower part of the video. So I'm not able to view 100% video in full screen mode. Does anyone have solution for this. (*Please don't suggest watching videos in YouTube app because i am trying YouTube app alternatives just for one reason."I can't auto trun off captions in YouTube app even though it's turn off in YouTube app setting as well as on web in YouTube playback and performance setting. So every time i click on new video i have to manually select setting and turn off caption."