GNOME 3.8 to drop fallback mode

Please don't interpret this the wrong way, but I think you may be confusing Xfce with fvwm, twm, or one of those. I actually don't know where Xfce keeps its configuration files, and have never manually edited them. I'm pretty sure they're somewhere in my home directory, because they persisted across multiple distro wipes, but I don't know what they're called, and don't much care.

When I do want to change something, I use the control panel, accessible from the bottom menu bar. You can also right-click on various things to get configuration menus that way. It's basically the same as GNOME2 as far as configuration goes, except GNOME2 had that horrible registry thing, and Xfce just uses files (I think?) But that's just an implementation detail.