Previously, in Firefox versions 46 and earlier, opening a new window (e.g. with Ctrl+N) would let the window manager position the window. This would allow the window manager to, for example, position the new window in empty space beside an already open window. In Firefox 47, Firefox will only position windows on the far left of the desktop. Unless windows are manually moved, they are placed all on top of each other.

Note that this is completely separate from the repositioning of windows that Firefox will do when starting up and restoring a session. That behaves the same in Firefox 47, with windows restoring to their original positions.