[Firefox] Insert cell before then after

[Tomasz Jakut]

Steps to reproduce

1. Open ​http://ckeditor.com/demo#full.
2. Open source mode and paste:

   <table border="1">
   	<tbody>
   		<tr>
   			<td>Cell 1.1</td>
   			<td>Cell 1.2</td>
   		</tr>
   	</tbody>
   </table>
   

3. Switch back to WYSIWYG mode.
4. Select both cells.
5. Click right menu button. Choose "Cell" → "Insert Cell Before".
6. Click right menu button once more. Choose "Cell" → "Insert Cell After".

Expected result

• The first new cell is inserted at the beginning.
• The second new cell is inserted after the first cell.

Actual result

• The second new cell is inserted before the first cell.

It seems to be caused by the fact that the selection's offsets after inserting new cell are not updated and pointing to the new cell instead of the existing one.

Other details (browser, OS, CKEditor version, installed plugins)

Firefox 48.0 on Ubuntu 16.04