[Firefox] Double line is shown after pressing Shift+Enter

[ijemmy]

Steps to reproduce

Can be reproduced in the CKEditor demo

1. Write anything in <p>
2. Enter
3. Backspace
4. Shift+Enter

Expected result

Expect one line to be shown

The HTML element should be

<p>

abcdefghijkl

<br></br>

<br></br>

</p>

Actual result

Two blank spaces is shown

If you inspect the element, you will see:

<p>

abcdefghijkl

<br type="_moz"></br>

<br></br>

#<br type="_moz"></br>

</p>

# /*the selection range is here*/

If you press backspace after step 4, one line will be removed. But when you begin typing again, your cursor will jump back to the first line.

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

Only Firefox. (Chrome/Safari are ok) Tested in OS X Elcapitan (10.11.6)

Can be reproduced in the CKEditor demo in CKEditor.com