Hundreds of thousands of you used Page Shot during its time as a Test Pilot experiment. Enough people that we decided to make it a part of everyone's Firefox experience!
Page Shot has been renamed Firefox Screenshots and is part of Firefox 56+. If you are using Firefox 56 you'll see the familiar button. If you are on Firefox 57+ you'll find Screenshots in the URL menu:
If you used Page Shot, your shots are still available (unless they've expired), and your Page Shot account has been migrated over to Firefox Screenshots.