Steps to Reproduce

• Create one 3x3 table in a richtext field
• go to the middle most right cell
• Add one cell after (This will create another column with the cell and not exactly another cell)
• A new cell is created in the right, along with a new column. Now try to access the first and last cells on that column (the red cells in the attached screenshoot). We verify that we can't.

Edit: Only happens on firefox. In chrome, a new cell is indeed added, but in firefox it seems that a whole column is added instead of only a cell. I've added a screnncast