Many websites like youtube, hotstar use the "F" key to turn window to full screen. Since Firefox has a shortcut "F" key to turn on find on page, when we press F key to make window full screen it turns on Find on page instead of making window full screen. Please take necessary steps to solve it. I suggest removing the F key shortcut to use Find on page since there's Ctrl+F shortcut to use Find on page.
Thank you, It's Krishnakant Garabadu from India