Press F11 to exit full screen

Thank you for your reply but this did not work. When I open Firefox which I am using with Norton Home Page extension I do not get the bar at the top with minimise, restore down and close. I have to open a new window using Ctrl + N then I get the bar at the top. Any help would be appreciated.

ah, another time where norton is the culprit, i recommend you dig around norton settings and "force off" its tampering with the windowing system. its something microsoft actively fight against, and does cause more problems than its worth (internal API calls and other under the hood stuff). uninstalling norton and using windows defender thats already deeply integrated into windows is an option, but in any way, a "security suite" should NEVER change the core function of the OS.