Running Firefox on my Microsoft Surface causes letterboxing even though it is a widescreen but only when in fullscreen mode playing videos on YT or Browser Games

Please help me as i want to record some gameplay but it keeps getting squished & it is pissing me off. I have asked on reddit & people were unhelpful & telling me to just buy another computer. this is annoying as it is causing me when playing games it causes text to be unreadable at times. Thanks in advance.