How do you exit fullscreen mode?

If you have a USB keyboard

Simply press F11.

If you have a touch or type cover

Pressing Fn + Page Up is the equivalent for F11.

If you have no keyboard

Press and hold (your finger) anywhere within the page to reveal the context menu. This pops open the top navigation elements as well and allows you to exit fullscreen mode.

Drag your finger to the upper right corner and an exit full screen (F11) will appear for you to click on.

In Firefox click the upper left corner. It's quite precise so I clîck it a few times to get it.