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 are a number of fancy animations enabled by default in the Proton UI. One of them is the download animation. If you have disabled animations earlier, you might want to get rid of the download animation too.