In CKEditor 3.6 the concept of toolbar button groups was introduced to enable faster and more efficient navigation using the keyboard or assistive devices. In all previous versions of the editor, the Tab and Shift+Tab keys had the same effect as using the Right and Left Arrow keys and were used to cycle between consecutive toolbar buttons.
Since CKEditor 3.6, Tab and Shift+Tab navigate between toolbar button groups, while the Arrow keys are used to cycle between the buttons within a group.
In order to change the new default toolbar navigation mode and use the Arrow keys as an equivalent to Tab and Shift+Tab, use the following toolbarGroupCycling configuration setting:
config.toolbarGroupCycling = false;