Caran, it's a feature for private browsing, here's what it says
"You've gone incognito*.
Pages you view in this tab won't appear in your browser history or search history, and won't leave cookies behind. Bookmarks you create will be preserved, however.
Going incognito* doesn't affect the behaviour of other people, servers or software. Be wary of surveillance by secret agents or people standing behind you. Learn more about incognito* browsing.
*Cookies are not cleared until you close all incognito* tabs."
There's a litte square top RH corner, slide it left and you'll go to the ordinary screen.