PLEASE NOTE THAT: This may not be the solution to your problem! There are MANY reasons why videos may.
You may experience YouTube fullscreen problem regardless of the browser that you are using. Here are tips to fix YouTube fullscreen not.
To switch to full screen, click the full screen icon in the bottom corner of the video player. To exit out of full screen, press Esc on your keyboard or click the Full.
Access full screen on youtube - mahjong