[FF] Difficult to add content after a new horizontal line

[damo]

Open the CK Editor.
Enter some text
Click on Insert Horizontal Line icon.

Result: The horizontal line is inserted and the editor focus is lost. It is difficult to place the cursor below the horizontal line.

Expected Result: The focus should remain in the editor and the cursor should be placed after the horizontal line. This is how it works in IE.

[fredck]

DUP of #4072