This is probably a UI change only. I have preferences set to "Check for updates but let you choose to install them". At Firefox 56.0 I was informed via a dialog box that an update was available. I clicked to "download" it, but it was not clear how to install. After restarting Firefox, it had been installed automatically. The dialog box should be changed to make clear that "download" means install as well, at restart if this is correct. Incidentally, the dialog forces an immediate response unnecessarily by permanently obscuring the top-right corner of the web page. This could be filed as a separate bug request.