As you may know, Firefox 57 comes with a new user interface, known as "Photon". It is intended to provide a more modern, sleek feel that’s consistent across multiple platforms. It replaced the previous "Australis" UI and features new menus, a new customization pane and tabs without rounded corners. There is a special option you can enable to drag the app window quickly. It is called "Drag Space" and appears as a tiny line above the tab row.