You may want to reset the mouse operation settings, background color of the 3D view etc... to the initial state on installation. In this case, you can reset your user settings by using one of the following methods.
Select [Edit]-[Preferences...] in menu and click
button at the bottom left of the Preferences dialog.Those settings are saved in a following file.
If FreeCAD is runnning, close it once. And after deleteing the above file, restart FreeCAD. The new, initial state user.cfg will be created and the FreeCAD user preferences will be reset.